Strange Drivetrain Noise

[SCFFR]

It's been quite a while since I have posted anything but have been enjoying my MK4 roadster for the past several years and working on some other projects. All was fine until I recently noticed a strange noise in the drivetrain. I guess it would be best described as a "clunking" sound which I first noticed going around a sharp curve in the road but it didn't happen all the time. When I got home, I checked the rear end for any loose bolts and/or nuts but didn't see anything that was obviously wrong.

The next time I took the car out, I pulled out of the garage and was slowly going down the driveway in first when I let off the gas and heard an even louder clunk. I parked the car back in the garage and again looked for any signs of what was wrong. Again, nothing obvious.

This morning I jacked the rear end of the car up with the intention of not stopping until I found out what was wrong. I checked the engine mounts, transmission mount, lower rear control arms and upper arm (3 link setup). I grabbed the driveshaft and gave it a twist to check the u-joints and they looked fine. With the transmission in neutral, I turned the driveshaft to check for any play before the rear tires started turning. Looks to be about 1/8" - 3/16" of play which I don't think is abnormal. The bolts holding the driveshaft to the differential are tight. I then checked the lug nuts on the rear wheels and the ones on the right side could be tighten just a little more. Thinking this might be the problem, I drove the car but the clunking was still there and is now more frequent. Even when I let off the gas/get back on the gas when I shift gears, I can hear the clunking noise which is definitely coming from the rear end drivetrain area.

Really not sure what to check for next. Note that the pre-98 Mustang 8.8 rear end was completely rebuilt with new axles, bearings, seals, 3.31 gears and Ford traction-lok unit. Unfortunately the person who rebuilt the rear end is no longer at the shop where I had it done and he was their "expert". I live in a rural area so its probably at least a 100 mile trip to a competent shop plus I don't like the idea of leaving my car with someone I've never done business with. If I knew for sure it was the differential, I could take it out of the car and just take it for repair.

I guess there is a chance that it's something wrong with the output shaft on the TKO 600 transmission but it was a new unit so my first guess is the differential. I'm thinking it just wasn't put together properly. If the pinion bearing nut wasn't properly torqued would this eventually cause the clunking?

Any ideas, recommendations or suggestions as to how to narrow down the problem?????

As always, thanks for your help and advice.
Ron

[Railroad]

Since you have to start somewhere, see if you can move the pinion flange in and out on the axle. You might need a small pry bar or screw driver. Should be no movement.

[GoDadGo]

We've had a similar issue with my brother's old T/A.
It was the pinion loosening up as Railroad had described.
We removed the nut and Re-Torqued it using Blue Locktite on the threads.

[Jeff Kleiner]

Here's another thought: ring gear to carrier bolts. Happened to me---lacking the tools the ONLY thing on the car that I did not do myself was change the ring & pinion. I sent the rear end to a friend's shop and his guy who did the work didn't use any sort of thread locker on the ring gear bolts A few thousand miles later they loosened up and then clunk, clunk, clunk when getting on or off the throttle.

Jeff

[skidd]

The first thing that came to my mind was a broken tooth on one of the sun or planet gears in the diff. Happened to an old F150 I had years ago. On corners it would make a somewhat consistent "clunk".. and some times (though rare) in a straight line under power it would "clunk" also.. I assume the missing tooth just happened to be lined up at the time.

[David Hodgkins]

Do you use a clutch style locking diff, like a Torsen?

Mine's been clunking since the day I first go-carted. Drove me mad for a bit but it seems to be a characteristic of some limited slip diffs.

[AC Bill]

If the pinion bearing nut wasn't properly torqued would this eventually cause the clunking?
Any ideas, recommendations or suggestions as to how to narrow down the problem? As always, thanks for your help and advice. Ron

Yes, if that nut became loose, it could be the problem.

Not many years ago, a clunking sound as you describe, would probably have quickly been diagnosed by FFCars members, as a u-joint hitting the e-brake cable bracket on the frame. It was a common issue, although usualy that clunk was also heard, when going over a bump.

Couple more things maybe worth checking, before you tear it down....

An loose item in the trunk rolling around, is probably too darn obvious..but hey.. it has happened to builders..

-Loose rear brake pad? Sometimes it can be that simple..
-Fuel tank's internal baffle broken off, and sliding back and forth in tank? ...That issue has "baffled" a few builders over the years..
-Rear lower control arm bolt loose, or worn/torn bushing?

Does it only happen on deceleration, or does it clunk on acceleration as well?
Can you recreate the identical sound, by leaving it in gear, engine off, e-brake off, and rocking the car forward and back?

Can you run it on a hoist, or jack stands, rear wheels off, and have a helper working the clutch, and throttle and brakes, while you try and narrow the noise down better.

[HCP 65 COUPE]

If the pinion crush sleeve is lose it would also gear whine pretty bad, maybe even leak gear oil(pinion seal) and destroy it self pretty fast.

[Railroad]

On and Off throttle clunk can be drive line or the rotation of the axle. The rod ends can transfer a lot of noise if they have any play in them. I would check all the lock jam nuts on any rod ends or adjusters. You need to do this with the suspension loaded. If you can get on a drive on lift it will be a big help. Sometimes muffler shops are not real busy and will let you in for a token amount.

[SCFFR]

Many thanks for all the replies.

While I was under the car the last time, I did take a small pry bar and put it under the pinion flange to see if I could move it in/out but it didn't budge. I had also checked all of the fasteners on the control arms and all were tight. Haven't checked the brake calipers but will do that next.
By accident, I did discover that the driver's seat bolts were loose and the rear of the seat had about 1/2" of play. I thought that maybe this was the problem but you would think I would have felt the movement in addition to the clunking noise.
Whatever is the problem, it happened slowly over time since it was fine the first thousand miles so I'm thinking that some kind of fastener has loosened (like on the ring gear) or some gap or tolerance was incorrectly set.
At first, I immediately thought a bad u-joint since I have had this type of problem in the past and it was easily diagnosed and fixed. The driveshaft is a new unit I got from Mark at Breeze Automotive which was made by Denny's and is a quality piece.
I think I will jack up the rear of the car and put it on jack stands and have a friend put it in first gear and play with the accelerator while I look underneath. Maybe by chance I will see something.
Thanks again,
Ron

[CraigS]

"I think I will jack up the rear of the car and put it on jack stands and have a friend put it in first gear and play with the accelerator while I look underneath. Maybe by chance I will see something." Be REALLY careful doing this but I agree it's a good idea. Pull the parking brake on and simulate pulling from a stop in 1st and then in reverse. Many times you need to load it up more than you can by hand. If you can get it to make the noise get a piece of hose like heater hose or even garden hose maybe 4 feet long. Tape one end to a yard stick. Hold the other end against your ear and position the yard stick end near the diff, the drive shaft, etc to narrow down the source. I recommend the long hose and stick so you don't need to be under the car.

[Just puttering]

Pull the parking brake then carefully tension the drive train in first, let the clutch out slightly, then reverse, do you get a clunk? Did you pry on the drive shaft u-joints. You say its new but it could be the problem, dont just try twisting with your hands. Is there any end play in the axles. If the c-clips or buttons on the axles or the cross shaft wore you could have play in the axles that would be easy to miss with disk brakes.

[SCFFR]

Thanks again for the suggestions. I drove the car out of the garage yesterday on the outside chance the clunk was because of the loose seat bolts. Clunk is now worse and continued for several seconds after I put it in reverse to back the car into the garage. I'm thinking that the new traction lok unit has gone bad. I'm going to drain the oil, check it for any metal and pull the cover plate off and look at the gears while turning the rear wheels. Unless it's something obvious like loose ring gear bolts, I think I'm just going to pull the rear end and take it to a speed shop in NC that specializes in late model Mustangs and let them go through it.

Will let you know what I find out.

Thanks again for your help,
Ron

[Mark Dougherty]

I am with Jeff on this one the ring gear bolts have come loose.
or the cross pin bolt is loose.

ring gear torque is 80 ft. lb.
and use red loc tight.

[GoDadGo]

I am with Jeff on this one the ring gear bolts have come loose.
or the cross pin bolt is loose.

ring gear torque is 80 ft. lb.
and use red loc tight.

I'm piling on too.
Pull the differential cover and see what has come loose.
Good Luck From The Swamplands Of Louisiana!

Steve

[Just puttering]

If its a trac-lock with no posi additive it will clunk when loading the rear when turning, they need posi additive!

[SCFFR]

Went ahead and drained the oil from the rear end and removed the rear cover. There were some very small pieces of metal/powder on the magnetic drain plug but nothing in the gear oil or laying in the bottom of the housing. As mentioned before, there's a little play in the differential but nothing major. The only thing I can find that "clunks" was the brake rotors but that was with the wheels removed so that doesn't count. All of the ring gear bolts are tight. Since I really don't know exactly what I'm looking for, decided to go ahead and remove the rear end and will take it somewhere to have it looked at and repaired - if there is actually something wrong with it. If not, I'm really at a loss as to what is wrong.
At this point, I would be happy to find out the Traction-Lok unit was bad and needed to be replaced just to have it fixed.

Waiting on my neighbor to come over and help me get the final couple bolts out so we can roll the rear end out from under the car and load up into my truck.

Thanks again!
Ron

[Railroad]

No harm in opening the diff up, but I would pay attention to the axle attachment points, ie bolts, rod ends, shock mounts etc.
Might as well open the U joints up and see how they look.

[HCP 65 COUPE]

Also check your suspension mount points for oval shaped holes and cracks around any of the suspension mount brackets on the frame.

[SCFFR]

I had drained the gear oil and pulled the rear cover but didn't see anything obviously wrong. I pulled the rear end and took it to a speed shop in NC that specializes in later model Mustangs. I explained the problem to the owner who said he would go ahead and take everything apart and inspect/measure every part and replace whatever was necessary. He said it might not fix the clunking noise but at least I could eliminate the rear end as the problem. Hope to pick it up this week.
When I was removing the rear end, I checked every fastener, bolt, mount, bracket, etc. and everything was in perfect shape and properly tightened.
At this point, I hope it is something in the rear end. If not, not sure where to look next.
Thanks.

[CraigS]

Ask the man to verify it is a Ford traction lock unit. I say this because my experience w/ a Eaton TrueTrac in my FFR and the Torsen in my wife's 2015 Mustang is that the gear type lim slips have an excess amount of slop. I assume it is because there are a lot of gears in there and each needs just a little free play. So all those free plays add up. Did yours look like this w/ the large pin and the "S" spring?
http://www.jegs.com/i/Ford-Racing/39...FRdWDQod0TEKdw
This is the clutch type unit that should be quiet in on/off throttle movement.

[SCFFR]

Update.
Got the rear end back from Fastlane Motorsports. Guys there were great. Opened it up and did a thorough inspection but didn't find anything wrong that would explain the clunking noise. Installed the unit back in the car, filled it with oil and took it out for a test drive. Still getting a clunking noise when I let off the gas in first gear but not near as bad. Noticed that there is no clunk when I let off the gas if the car is in reverse.
Really don't know where to look now. I think I'm just going to drive the car and see what happens.

[walt mckenna]

Update.
Got the rear end back from Fastlane Motorsports. Guys there were great. Opened it up and did a thorough inspection but didn't find anything wrong that would explain the clunking noise. Installed the unit back in the car, filled it with oil and took it out for a test drive. Still getting a clunking noise when I let off the gas in first gear but not near as bad. Noticed that there is no clunk when I let off the gas if the car is in reverse.
Really don't know where to look now. I think I'm just going to drive the car and see what happens.

I was in the differential camp until that came back clean. Many suggestions about suspension links and your inspection of them would lead to dismissal of a suspension problem, but I would re-visit that area. You seem to have experienced a definite change after you removed and replaced the entire differential which means you disconnected and re-connected all the suspension points with a resultant change. I would focus on the adjustable third link itself, the third link pivot points and the link to housing connections. My first clutch was too heavy duty for the car and very grabby which made the adjustable third link jump back and forth producing driveline chatter. My only change to eliminate the problem was to install an OEM style clutch with organic friction material both sides of disc.

[myjones]

Based on the prior events the above suggestion is a good one to sort through. I wonder if a Heim joint has worn out the inner race. With the car load on it
you might not find any play but remove them a joint at a time and check each one for a sloppy fit, if one is different you will feel it when it not loaded.
HTH
Dale

[SCFFR]

Thanks for the suggestions. The owner of Fastlane also commented on the Heim joints so I carefully inspected them when I was installing the rear end. They appeared to be fine in that the ends weren't sloppy or loose. I guess I could replace them just in case. Does anyone have the specs on the two ends? Just puzzling since the car only has about 500 miles on it.

[CraigS]

Have you checked the three bolts that attach each traction bracket to the axle? They need to be really, really tight. The holes are just slightly oversized which will allow a tiny bit of movement. Admittedly it was because I autocross on R compound tires but I finally welded mine to the axle.

[SCFFR]

Thanks Craig,

Like you, I also welded the arm to the housing in addition to the bolts so don't think that is the issue. I had also checked it when I removed the rear end.
If I can find the specs on the two Heim ends, will replace them and see what happens.