Odd transmission rattling noise
 

My '86 shifts nicely and I think it is behaving as normally as a 915 can; the upshift from 1st to 2nd is a little harder than the others, but I understand this is pretty typical. No grinding noises. Shifting the car is for me one of the funnest things about driving it.

Here's the question: If I accelerate fairly hard in any gear starting from low RPM (1500 or so), I hear a sort of rattly noise in addition to normal engine noise. Hard to describe ... not grinding or real noisy, but noticeable especially with the targa top on and windows up. At higher RPMs (>3000) it pretty much goes away. If I don't accelerate hard, it is not very noticeable.

Is this normal for a 915, and if not, do any experts out there have ideas on what may be the root cause? Thanks in advance; I highly value the good advice I have received from the generous folks on this forum in the past.

-Jeff

Are you positive it's the transmission? Could it be from the engine? How many miles on your engine? My out of left field guess is cam chains rubbing on the guides. You car has pressure fed chain tensioners and it could be that they aren't keep the chains tight enough as lower rpms (lower oil pressures).

The fact that it's rpm dependent and not gear dependent steers me to the engine. I'll definitely defer to the experts but cams chains was the first thing that I thought of. -J

Good point, I don't know for sure the origin. If it were the engine, would the sound be load dependent? It is loudest at full throttle, low RPMs (~2000), under load. No rattle at no load.

The sound is sort of like a coffee grinder.

I have an old Braun coffee grinder that sounds like a buzz saw. Is the noise like a chain dragging on the ground? Like a rattling that goes away after you hit a higher RPM point?

I bet the noise is your transmission under load without the torque to spin it quickly enough. I have no idea what it is but it is normal. Maybe some sort of mechanical cavitation (making stuff up here).

My 915 does it after a Mayo Performance rebuild and I have been in an 89 930 with a spiffy G50 five speed that does it (20k K mile garage queen).

I am struggling with first gear causing the noise at 1500 rpm, though.

Thanks for the insight, Bob. The noise is in fact like a chain dragging on the ground, and it goes away at a higher RPM point. I was unclear I guess, because it doesn't happen in first gear ...... not enough time at full throttle to notice it! ;)

Interesting that it may be more or less normal for these transmissions.