Clunk sound coming from the rear

[83gt351w]

So I took the ole girl out today for her maiden voyage. Felt great even if it was only 1 mile. But, there is a nasty clunk sound upon starting and stopping. The driveshaft is good, that just came back from the shop. The rear end was rebuilt by myself, it does have the Cobra clutch pack in it that I heard can make popping sounds until it gets broken in. The rear suspension is also all new as well. This is where it gets strange. Before all my work, the car had factory suspension, with an automatic trans, and a bad rebuilt rear from a “friend” and shifting into drive or reverse, that same sound was there. Any ideas? It does have friction modifier in it.

[2nd chance cobra]

Did you check pinion backlash? And your trans to driveshaft play. Make sure your driveshaft slides easily in the trans. Check trans mounts.Check all your rear end mounts, car side and rear end side. Without hearing it, it's gonna be tough guessing. Check the obvious stuff first, brfore we go chasing a unicorn.

[BMW Rider]

Often if you have too high an idle speed it can cause a hash clunk when you put it into gear with an auto trans. If the idle speed is high enough that the torque converter is engaging, it will apply torque to the driveshaft on engagement which will take up the normal slack that is in the entire drivetrain causing a clunk.

[homer302]

First, I will start by saying that any part that "just came back from the shop or ""rear suspension is all new so we know that can't be it""..." Is IMMEDIATELY SUSPECT, not assumed to be good just because it came back from a shop or is new. That being said (and the fact that you say the noise existed prior to any work).... I (essentially) was the original owner of a '98 Cobra That had a clunk when letting out the clutch to move and when stopping and also when going over speed bumps. Gave one of my smaller friends enough beer to climb in the trunk and let me shut the lid and drive around while he tried to locate the noise. Turned out to be an upper control arm bolt on the frame end that was never tightened properly at the Ford plant. I'm not talking loose. It was tight and likely would've lasted the life of the car, it was just THAT much too loose and allowed it to move. I found the torque spec and it was not far off but enough I guess.
It sounds like you can create this noise while totally stopped just by dropping the car in gear. That could be a worn transmission or motor mount. Either allows the drivetrain to move too much, causing it to hit the chassis of the car. If you could provide just a bit more detail about exactly how and when the issue is maybe it might help. The more times I read your post the more it sounds like a broken motor mount.

[wraithracing]

If you jack up the rear of the vehicle and place the jack stands under the axle tube. You can run the vehicles up thru the gears while listening for noises, etc. If the noise is not present when place on the stands, then there's a good chance its a loose suspension bolt or bad bushing. If the noise is still present, then you should be able to narrow it down form there. Good Luck!

[Lowetlx]

Did you grease the splines of the slip yoke?

[83gt351w]

Update.

Got the car car up to temp, placed the rear on stands, and shifted through the gears. At first, I couldn’t find anything. Then all of a sudden I heard it. Crawling under the car, it sounded like it was the front of the driveshaft, going through all the gears again, it moved to the back. Under the back, watching the diff, I noticed the pinion collar try to “jump” towards the bottom of the car, and that metallic “clunk” was there again.

This is the same driveshaft ive always had. It’s the aluminum Ford racing one. Is it possible, all the years of sitting, the u joints are kinda “frozen” in a certain spot? Before I installed it, the yoke, and the tail wouldn’t flop around on there own, but moved easily enough by hand. The shop was asked to check the driveshaft for balance, but if the u joints were suspect, wouldn’t they have mentioned that?

[wraithracing]

Are you sure your Pinion bearing pre-load is correct? Sounds a bit like there is slop/play in the pinion due to either the crush sleeve not being properly crushed and the bearings pre-loaded or the crush sleeve has collapsed and you have too much play and no bearing pre-load.

A bad U joint can cause some of the symptoms you are describing and are definitely a possibility, but the pinion should not move up or down at all even with a bad U joint. I would grab the pinion flange and see if you have any movement up/down and/or side to side. If so, then something is wrong with your pinion set and will need to be addressed.

Generally if you have too much play in the pinion you will not see it under load as the pinion will walk forwad into the housing under load, but as soon as you let off the pinion will try to walk back towards the ring gear and cause the clunking and/or cause damage to the pinon if it contacts the differential. Good Luck!

[83gt351w]

Trey,

Hopefully it is just my post not being clear. After finally crushing the collar, about 400 ft lbs of force, I continued tightening it to I believe it was 24 in lbs of rotational force.

By pinion flange, I mean the front of the diff tries to jump up towards the floor board of the car.

[wraithracing]

Trey,

Hopefully it is just my post not being clear. After finally crushing the collar, about 400 ft lbs of force, I continued tightening it to I believe it was 24 in lbs of rotational force.

By pinion flange, I mean the front of the diff tries to jump up towards the floor board of the car.

Hopefully everything is set up correct and it sounds like you did the process correctly. I am still suspect if the front of the differential is moving upwards. Possibly a binding u joint, but I am not sure.

At this point, it might be worth removing the driveshaft from the rear axle and verifying there's no play in the pinion and then check your U joints in the driveshaft.

The only other thing you might also double check would be your driveline angles to verify they are matched. If by chance your angles are not matching up that could be causing a binding in the U joints.

[83gt351w]

The pinion flange has zero play in it except the standard grab the driveshaft and can turn it left to right with about an 1/8” play, which I know you’d hear a little clunk, but that’s not what I’m hearing. I pulled the shaft and removed the u joints. Seemed a little catchy in the rear. I’m not gonna sweat bullets yet, unless this doesn’t fix it. As for pinion angle? Hopefully that’s easy to fix.

Joys of rebuilding cars, Trey

[Lowetlx]

Sounds like slip yoke boom. If they the drive shaft shop cleaned the yoke and the splines were not greased when you reinstalled it then you will get that noise. Ford had a tsb with a special grease years ago to correct it.

[gr79]

Mine did that.
Parked, rocking car back and forth made diff nose jump up.
Clunking rr on accel/deaccell, car pulled rt/lt on-off gas. Wore out rear shock several times.
Intermittent rattle like tailgate was not latched down.

PS LCA front bushing was shot.
Replaced arm. No more dff jump. Car stays straight. No clunk.
I suppose a pinion bumper would had stopped it some.
One of these days will retrofit it. Been in parts box for years.

This noise was more like a knock.
U joints clink.

[83gt351w]

The slip yoke does not have grease in it at all. Just reading up on that, it appears it’s for reducing the knock you hear when shifting. How full do you pack it?

[Lowetlx]

Not a ton just a scoop with a couple fingers and push in all the splines

[83gt351w]

Helping the BIL remove the trans from his Vette, we proceeded to talk about the Mustang. Not mentioning what Trey has said, (he helped with the rear diff, pretty savy car guy) he said as well to proceed with new u joints, and asked about the pinion angle. He said it would only take a couple of degrees and the u joints could bind. I guess they need to match each other. The car has an aluminum headed 351w so it’s a little heavier than the 302. The suspension hasn’t had a chance to settle yet, I’m wondering if that would help. Which circles me back to Trey saying not to torque the suspension down, and go for a short run, and tighten it to specs. I want to get a few miles on it to see if it works itself out, but this clunk is extremely loud, to the point you think it’s gonna break something. Updates to come

[83gt351w]

Update

The BIL talked to the driveline guys at his work. Their first response was replace the u joints, and we’ll go from there. Well, the BIL brought over some Spicer u joints, we installed them, drove the car for 30 minutes out in my wavy 2 acre field, and that dang thing never made that noise again. I did however have a couple other issues pop up. The car bottoms out in the rear, H&R Super Sports, easy fix. Just roll the fender lips. And I have a weird metallic rubbing in the bellhousing area. Doesn’t sound serious, just what comes with the ground up deal.