Gearbox rattle noise on overrun 2.5-3K

[-Ollie-]

Hi guys, I have an unpleasant noise in my M3 I am keen to identify.

The car is a track car, fully stripped out with a standard dogleg gearbox which was recently rebuilt. Flywheel is LTW (unknown brand) and clutch is also unknown, but nothing too serious.

As the revs drop to around 2500-3000 rpm on overun, I am hearing a terrible resonance/metallic vibration noise - sounds like rocks in a steel bucket. It's really quite loud and unpleasant. As soon as any load (power) is applied, the noise immediately disappears. if the revs are kept above 3000, or below 2500 there is no sound.

Does anyone have any ideas of what the issue could be? Gearbox mounts are E28 M5 items. The noise is definitely originating from the gearbox/bellhousing area and is not from the shifter assembly. Shifting is perfect, and no usability issue to report, just an awful noise!

Thanks in advance.

Ollie

[Dave @nz]

I think draining the oil from the gearbox and inspect the oil for any debris is the first thing to do just incase there is something going wrong internally.

[Ant's m3]

might sound silly but i would also check the bell housing hardware hasn't come loose

[-Ollie-]

It's odd, I've done over 1500 v.hard (some competition) miles with it like this - if something was eating itself surely it would have let go? I have an event this weekend so will drain the oil after that.

All external hardware has been bolt checked and is tight.

[M-tinker]

My car has the same gearbox noise at a similar rpm range. Only happens when off throttle or coasting. Only at that narrow 2-3k rpm range. It sounds like metal grinding or a worn bearing. Not pleasant at all.

I’ve checked the tranny plug. No significant metal shavings. I’ve tried different tranny fluid and different weight. I’ve changed to a different diff, different tranny mounts. The noise is still there. The tranny shifts fine and the noise goes away as soon as you add little throttle to load up the drive train. I’ve learned to live with it for years, avoiding the noise by using throttle, choosing a different gear, or clutch in early when coming to a stop. Knock on wood the old tranny is still running strong.

What flywheel and engine mount are you using. Mine is JBR and 740i mounts. I wonder if they have something to do with the noise..

[-Ollie-]

M tinker that sounds just like mine. I too have swapped the diff out to no avail. Engine mounts are vibratechnics and gearbox are e28 M5. Have you ever got close to working out the cause?

[-Ollie-]

EDIT: my flywheel is TTV I think, perhaps it's just chattering noise from this?

[M-tinker]

Not really. The next thing to try is to go back to stock flywheel. My JBR one clatters the drive train on idle and low rpm. It might have something to do with the noise at higher rpm. My theory.

[-Ollie-]

Not really. The next thing to try is to go back to stock flywheel. My JBR one clatters the drive train on idle and low rpm. It might have something to do with the noise at higher rpm. My theory.

Thanks Tinker, I'm swapping my motor out over this winter and will be changing the flywheel then also. Will be interesting to see if my noise remains afterwards. I did speak with my engine & box builder over the weekend about it, and they said it's as a result of a very light flywheel and nothing at all to be concerned about. Just sounds terrible! Happily most of the time I've got a helmet on when driving the E30, may need to keep the noise cancelling headphones in the car for the occasional road trip now too!