Rear clunk, have you fixed it?

[grandmaster]

I'm posting this in desperation that someone may help. We have a 2000 328CI, steptronic, with 50k miles - gently driven. When driving at slow speeds, under 20 mph or so, if you hit the gas slightly or let off the gas, shift from 2nd to 1st in manual mode, there is a rear thud clunk noise (rear drivers side). Now, it gets more strange since it will not occur in reverse, over rough road, shifting from drive to reverse and will ONLY occur after you drive it for about 30 seconds or more...(giving something a chance to warm up or move around a bit??)

Here are the things we have replaced:

-Output flange per the service bulletin
-Trailing arm bushings
-The big differential bushing on the side, but NOT the top two (not in stock)
-The differential iteself (the old one had serious wear for some reason)
-Driveshaft w/ center bearing
-Transmission fluid
-Lots of inspections for subframe damage

What might some other culprits be? Three dealerships and many mechanics later, nobody can find the problem.

Thanks for ANY help.

[BMerViXeN]

Wish I had the answer to this one. It's sounds like the same issue I'm having with my 2000 323i. Does it soudn like "loose screw" or "washer when you drive?

[gotKRAUT]

on a keeping it simple note....have you replaced the Rear Shock Mounts? they clunk like described when bad....

[grandmaster]

The noise is a clunk when tapping or releasing the gas in a low gear, or downshifting from 2nd to 1st with the steptronic. No rattles when driving though.

[grandmaster]

Hey, no I haven't checked the rear shock mounts. But wouldn't they make a noise when travelling over railway tracks or rough roads?

[441tech]

I was going to say shock mounts, but normally they would make noise over bumps, railroad tracks, etc....could be the spring seats as well, since they utilize insulator pads.

I think you may have a torque-related issue. It's about the only thing left that makes sense, since you say it's primarily when you're pushing and releasing the gas pedal, and low-gear shifts. Virtually the whole powertrain shifts when you add or remove power from the engine, as does the rear axle. It IS possible that the shock mounts or spring seats have worn to make noise only when torque is applied, since this is also a common, rotational wear direction--whereas going over bumps and tracks is more of a vertical suspension movement. Part of the reason why BMW's are so smooth and vibration-free is because they pretty much mount the whole power and drivetrain on rubber insulated mounts, including the rear axle (diff.), transmission, etc.

If you're sure it's coming from the back, (or maybe underneath the car?) take it to a shop, and have the mechanics put it on a lift with one guy in the car. The guy in the car can start the car, put it in gear, depress the brake firmly, and simultaneously torque up the powertrain by stepping on the gas pedal a bit (This is also known as "power-braking"). That way, another guy under the car can have him repeat the cycle while looking, listening, and feeling around for your clunk.

I've figured out A LOT of mystery problems with vehicles over the years using this method.

Good Luck.

441Tech