Hi all,

this is on a W111 Coupe. When I cruise in 5th gear at 3000 rpm and 80 mph, I have that noise in the cabin which ends up driving me crasy on long distances. I have to go fater, like 85 mph, or slower, like 75 mph, for the noise to go away. Also the noise at 80 mph is much lower if I am at light throttle, and gets louder if I step on it.

- If I downshift to fourth gear and stay at 80mph, the noise is gone;
- If I slow down at 3000 rpm in fourth, the noise is not there either;
- If I go faster or slower in fifth, the noise is gone.

It is really the combination of 5th gear + 3000rpm + 80 mph that creates the noise. Change any part of the equation and the noise is gone.

I also replaced the prop shaft and rear end recently for other reasons. The noise was there before, and still there after, same way. So I think the rear end and prop shaft are not to blame. Engine at 3000 rpm in another gear doesn't make the noise.

Could it be the trans, and more particulatly when it's in fifth gear that would do that noise only at 80 mph? If yes, what could be the reason? Or may be a combination of harmonics at 3000 rpm and rotation of the trans/shaft/rear end at 80 mph that would get into some kind of resonance?

Thanks in advance for the help!