Ok. I'm used to all the usual noises and rattles from my 2013 M3W. There was a new one today that I'm having trouble identifying. At low RPM and speed there's an occasional slamming sound that seems to come from either the rear of transmission, drive shaft, or BB. It sounds and feels like a single hammer hit to the frame just under the seat. I can't seem to figure out what sequence causes it, but it has happened a few times. It's not the Centra compensator drive, nor is it the clutch.

Thoughts?

Could be slack in the belt.
I get it sometimes on gentle acceleration, after overrun, if you let the revs drop too low. The 3 W hates being lugged

Hi Matt
Possibly BB backlash, especially at low rpm when load changes. As a completely obscure cause if you have just noticed it , would be to examine the spark plug leads. If damaged have tendency to short or give emf bursts and give erratic low rpm snatch in drive train?
I recently changed mine as I found that they had cuts and damage from rubbing on the cowl/bonnet. Engine smoother at low rpm now. Mine possibly been like that for sometime
I got longer leads and rerouted and zip tied to ensure no contact with bonnet

I had a simular noise on my car.

It was exactly as you describe. took it over to factory twice and they came out with me in the car.

They reckoned it was ok ...just loose play in the BB. Not so sure myself that it was ok as it sounded like the mesh in the BB was way too loose or something had come adrift. Anyway the car drove just fine and so I came to accept that they were right.

Matt as P964 says its likely from BB if you are in too high a gear & you let the revs drop. If you get a judder snatch a lower gear.

Tim

Yes. I think its is backlash in the bevel box I have had 3 bevel boxes in my car and all have the same noise. Usually a loud single clunk when at slow speed over a certain type of bump or pot hole. Once it was very much more pronounced over a corrugated road surface on a trailing throttle, I thought the bevel box was about to self-destruct.

Interesting, now that I think about it, I did hit a small pothole. Could it be something related to the belt geometry as the rear axle swings up? Maybe the belt is loosens up momentarily and the slam is it re-engaging. I should have a look to confirm BB is aligned to swing axle pivot.

The output from the BB shares the same centre as the swinging arm, so at low drive loads (not accelerating hard) if the swimming arm comes up quickly (going over a bump) it has the effect of unloading the output from the BB and you get a clunk. Its completely normal. The belt cannot slacken as the wheel is fixed at one end of the wing arm and the drive pulley shares the shame centre.

I never experimented that problem before having the NVH system fitted. I complained several times about this characteristic noise occurring only when you hit a bump on the road, but never received any explanation. As usual we'll be condemned to suffer it without any solution to stop it...

You need to check you`ve not backed over SWMBO!!!!