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?

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech