Lately I've noticed an odd groan or hum. If the car is driven to speed and then slows down to the low 30s, if it is in top gear a harmonic humming or buzz occurs that is almost a vibration.

If I downshift, accelerate harder or once my speed goes up to around 40 the sound goes away completely.

As a process of elimination I immediately changed my rear axle gear lube, but that made no noticeable change.

I'm wondering about exhaust harmonics, driveshaft supports etc that might make the noise - any suggestions on what to try next?

 

You have the same problem? A vibration or noise from the rear at low speeds?

 

It could be motor mounts, a heat shield touching an exhaust pipe, maybe a torque converter lock up issue, wheel bearing?


Depending on what car you have, from 2003-2007, not exactly sure on the years, but coolant got into the transmission fluid and caused issues similar to what you were talking about. Benz calls it a buzzing or droaning noise that occurs in the higher gears at usually 30-40mph range and usually very light throttle.

 

That definitely describes the noise I'm getting, but I just had the transmission fluid changed with correct MB fluid and filter.

This is leading me in the direction of having my mounts inspected, along with the flex discs in the driveshaft (I'm assuming in a 1999 they still have those, yes?)