6MT Clunk When Accelerating in Gear

Just started noticing this clunk this morning. If I am coasting in gear (tried 2nd, 3rd, and 4th) and ease onto the throttle, an audible clunk comes from the back of the car near the differential.

It only seems to happen when I apply gas after coasting in gear. I've tried to make it stop by really lightly rolling onto the throttle, which only worked a couple times. Applying the gas at any normal driving rate will give me this clunk 100% of the time. Any ideas on what it could be?

The M3 is a 2005 with 60k miles. RTABs and Diff Fluid were replaced at 48k miles for Inspection II.

Subframe?

Could be diff bushing.

If its a persistent rotational clunking, on acceleration, it could be the center support bearing...doesn't sound like what tour describing though.

Yeah it's not a consistent clunking, just a single clunk when initially applying the gas. After that, it's fine. Diff bushing sounds like a good possibility though.

One thing I have noticed is that it doesn't make a noise when travelling at highway speeds in 6th gear. It only seems to be around town in 1st through 4th gears. And the sound occurs when I'm already in gear, coasting, and then re-applying the gas.

There are external torx bolts that secure the diff at least on (non ms)

I noticed mine became slightly slack, which caused a bit of a clunk/dinging sound. I tightened them really well, and the sound went away.

Its just a guess on my part, but will cost you about 20 min to check out.

Good luck.

Originally Posted by bbenj10

One thing I have noticed is that it doesn't make a noise when travelling at highway speeds in 6th gear. It only seems to be around town in 1st through 4th gears. And the sound occurs when I'm already in gear, coasting, and then re-applying the gas.

That is because the car is putting out very little torque in 6th at HWY speed. Our cars have suffered from weak diff mounting bolts and BMW, superseded to different bolts to correct the problem. There are three diff bushings, which these bolts run through. Two of them are toward the top on the rear cover, and are the bolts that tent to shear. There is one going horizontally in the front center. The bushing are a good robust design, so unless your up around 100k miles it would not be my first suspicion. Also while under there its good to look for subframe mount cracks. The most common and visible will be at the rear drivers side mount. You need to clean the area real well, grime and undercoating will conceal smaller cracks.

Edit- You will want to remove one at a time, so your diif stays in place. You need to look at this before driving any more, its a possible safety hazzard!!!

Dealer told me it is rear subframe mounts and differential mounts. They're saying it's a 13 hour job labor wise. Is that way high or about right for replacing all the mounts??

Originally Posted by bbenj10

Dealer told me it is rear subframe mounts and differential mounts. They're saying it's a 13 hour job labor wise. Is that way high or about right for replacing all the mounts??

Slam on the Brakes, with the Stealership!! The time they are quoting is not insane, as the subframe has to be removed to do this. I dont buy that all of your bushings are shot. If it is true it would be nuts not to put a reinforcement kit in while you are in there. Shoot me a PM with your email, and I would be happy to give you my personal advice.

Start with the diff bushing as its cheapest and easiest.

Research all the 40+ bushings on the e46, and plan on replacing them all, little by little, if you plan on keeping the car for a long time.