Originally Posted by jay_bob

Update on this:

Finally the car popped a code for wheel speed sensor left rear. ABS fail, ESP Fail warnings on the dash.

Replaced all 4 wheel sensors with genuine MB sensors (sorry got burnt too many times on aftermarket sensors), cleared codes, did the ESP drive test, and guess what, the buzz started decreasing.

Reset adaptations on the transmission and it's now nearly gone.

Wonder if the bad speed reference was messing with the transmission operation and the transmission was trying to compensate for the bad data?

Originally Posted by TimFreeh

Or maybe the ESP magic was manipulating the rear brakes trying to correct some perceived anomaly in response to bad speed signals?

Mercedes dropped the SBC brakes at some point in the 211, does your later Bluetec have SBC brakes?