Hi Gilly.
It seems that you are the man to ask. I've got a 99 ml430 with 100000KMS that has this howling/growling noise at 2000 to 2200 rpms. This is a problem after the engine warms. I've had the bearings checked by MB and weren't able to detect if that was the problem. MB machanic heard the howl while driving it but wasn't able to detect it while it was up on the hoist in gear at 2000 rpms. He determined that it was load related and would charge me hourly to reposition the engine and exhaust until the noise was gone. I can't afford to pay his learning experience. Please help