Clunk in the rear

[SteelGreyM]

Hi all,

Seems over the last 3 days my entire car fell apart. Drivers side window switch broke, fog light cover fell out, header nut popped off, and now I have a clunk in the rear.

I have always had some sort of clunk back there. Especially when I shift gears or put it into 1st at a stop and whatnot. I think that is typical M clunk stuff though. Now however, the clunk in the back has gotten much louder and more noticeable. If im in first gear, roll up to 6k or so and let off the gas quickly there is a thump that comes from the back.

My question to you guys is where should I start diagnosing? For reference, driveshaft, CSB, and guibo are almost brand new. And all bushings in the rear have 20k miles on them. I am running e36 m3 rear diff bushings in the rear diff cover (when I did my subframe job). I remember reading that those are softer than e46 m3. Maybe they are wearing too quickly?

Should I just get under there with a big prybar and start prying thing around?

[bimmer]

Check to make sure the diff mounting bolts are all still tightened to spec. Do you have the upgraded rear bolts? How many miles on the diff itself?

[SteelGreyM]

Check to make sure the diff mounting bolts are all still tightened to spec. Do you have the upgraded rear bolts? How many miles on the diff itself?

Will check those out. Yes I do have the upgraded bolts. Diff has 146k on it. I use BMW brand diff fluid changed every 30k.

[bimmer]

From what I have read, 150K miles is about all you get out of these stock diffs. Obviously replacing/rebuilding is a last resort, but that was what fixed my similar issue and my diff had similar mileage under it's belt:

https://nam3forum.com/forums/forum/main-forum/e46-2001-2006/30069-clunking-when-on-off-the-gas-quickly/page2

sapote has a good post in there about how to test for play in the diff gears. You could try that given much of your driveline appears to be new except the diff itself.

[sapote]

the clunk in the back has gotten much louder and more noticeable. If im in first gear, roll up to 6k or so and let off the gas quickly there is a thump that comes from the back.

This is very hard engine braking; I'm surprised that this didn't cause a noise before even with normal car.

Yea, check the diff mounting bolts first. But I think the clutch stack has worn out and caused more plays between the two side-gears and the spider gears, causing the clunk.

Pre 2005 car with version 1 diff: right wheel jacked up and left one on ground, tranny in neutral (post 2005 with version 2 diff, left wheel jacked up, right wheel on ground):
1) try to turn it and see how much force to make it turn around; this tells the clutch stack slippage.
2) rock the wheel slowly and not forcefully, and find out how much total free plays (inches) from stop to stop. This is the side-gear

[Phoenix///M3]

This is very hard engine braking; I'm surprised that this didn't cause a noise before even with normal car.

Yea, check the diff mounting bolts first. But I think the clutch stack has worn out and caused more plays between the two side-gears and the spider gears, causing the clunk.

Pre 2005 car with version 1 diff: right wheel jacked up and left one on ground, tranny in neutral (post 2005 with version 2 diff, left wheel jacked up, right wheel on ground):
1) try to turn it and see how much force to make it turn around; this tells the clutch stack slippage.
2) rock the wheel slowly and not forcefully, and find out how much total free plays (inches) from stop to stop. This is the side-gear

How much side play is considered normal? I have about an inch and a quarter at 98k.

Sent from my SM-G950U using Tapatalk

[sapote]

How much side play is considered normal? I have about an inch and a quarter at 98k.

I'm not so sure, but 1.25" is about the upper limit of normal. But this is not the main reason of the clunk, which is more profound when the clutch stack slipped causing the side-gear hit the spider gears hard on the left side for V1, and right side for V2 diff.

[heinzboehmer]

1) try to turn it and see how much force to make it turn around; this tells the clutch stack slippage.

Do you know how much torque should be required to get the clutch pack to slip? I tried looking for a torque spec the other day (both on the internet and on TIS) and couldn't find anything.

[sapote]

I don’t know the clutch slip torque spec.