Thud when accelerating

[BOOSTED]

Hi everyone,
My m3 is making a thud sound from the rear end, it only makes the noise when I press on the accelerator and when the car shifts gears. Does anyone else have this problem? And what would be the fix?

some background on what has been done to the car so far.
The rear subframe was just reinforced with cmp plates and the top cmp bar. I also had them change the Guibo, center support bearing, solid CMP subframe bushings, and the differential bushings were swapped for the revshift poly black (soft) bushings. After the install the front diff bushing bolt backed out 2 times then my mechanic reinstall the bolt with red thread locker and thud got somewhat better as its only present when I press on the accelerator hard, instead of all the time. Not sure if the is a problem with the bolt again or is it something else. The mechanic is saying it’s because of the solid subframe bushings

Here is a small clip with the sound present

https://youtu.be/KoFtVYnJlUg

[heinzboehmer]

That noise is absolutely not being caused by the bushings. Since the bolt backed out twice, are you sure the threads in the diff are good?

Recently did some very similar stuff to my car and found that the threads in my diff were destroyed. I had previously attempted a repair with a timesert but that failed (probably user error, not saying the timesert didn't work), so I had to drill the hole out and tap it for a much bigger bolt.

[GhostGrayM3]

My front diff bolt being loose caused the same noise for me. My Indy shop custom made a bigger bolt and the noise went away. Something about the hole becoming oval from the play from the bolt.

[mrgizmo04]

Does it also make that thud (maybe even more pronounced) when you engage reverse?

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[Kdubski]

Either diff bolts or... trust me on this one.... rear shock mounts. Rear shock mounts for what they are cause some loud noises when they go out.

[eacmen]

https://www.rogueengineering.com/Rogue-Engineering-E46-M3-Differential-Bushing-Front-Position-FIX_p_256.html

I don’t run it myself but know people have had success with it.

[eacmen]

If you can get a gopro on the rear of the car while driving might shed some light on what the issue is.

[heinzboehmer]

If you can get a gopro on the rear of the car while driving might shed some light on what the issue is.

Yeah this helps a lot. You could also put it up on jack stands and (carefully of course) try to recreate the noise while you film the underside.

[mrgizmo04]

I've done jackstands. Sometimes successful, sometimes not. DSC has to be off, and you have to put a load on it with the parking brake. Sometimes the parking brake is just not enough load vs breaking static friction of actual car rolling on the ground. So then just use a pry bar or something long and try to figure out what is loose by jamming it into places and applying leverage.

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[Mspir3d]

That’s not normal at all. If you paid someone to take it out and do the repair, I’d take it back and ask them to finish the job. The front differential bolts are notorious for backing out. I put a nord lock washer on mine after I had to timesert it. If you do try to trouble shoot it, make sure you also check the wishbones and control arms where they connect to the cradle.

[Onefastsicilian]

front diff bushing.. theyre known to back out.. easy fix.

[BOOSTED]

Thanks for all the replies, I just ordered the rogue engineering kit and spoke to my mechanic, hopefully that pesky front bushing fixed for good!
will also check the shock mounts as well as these were just changed when the coilovers were installed.

[Arith2]

A mechanic throws parts at cars and hopes it fixes them. A technician diagnoses the problem and intentionally fixes it the first time. This is more for the shop than you but...

Don't throw parts at cars.

[r4dr]

TIL I'm a mechanic

[Tones]

MCLUNK

Jack your car up. Grab the rear wheel and move it back and forth. If you have tons of play and hear the thudding when moving the wheel back and forth from your diff than your lsd clutches are worn out.

Racing diffs sells a rebuild its 450$

https://www.instagram.com/p/CIV3aWep...=10rt6z5f7mam2.

This is a video of the symptoms.



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