Ok folks, I've searched all over these forums (not just W164) and found some (sort of) useful info but not my problem, I don't believe.

Here's the deal, my daily driver, an '11 ML63 with ~240k km (145k miles) has had an odd vibration for a couple of years now that just completely baffles me and every mechanic I've had look at it. I'm now getting more serious about finding the culprit because it's getting worse, at first it was only extremely sporadic, it's now pretty regular.

The issue: at 1700 - 2100 rpm (and only) under under very light throttle there is a very pronounced vibration (like an air chisel against the frame rail) which seems to be coming from the area of the rear differential. It is worst at about 1800 - 1900 rpm and in any gear / speed combination. I say rear diff. because with a mechanic in the back we test drove it and reproduced the problem, he is sure it is coming from the rear diff. area. Definitely not the front diff, way too far back, also too far back to be the transfer case.

So far I've drained, flushed and refilled both diff's and the transfer case twice, no signs of contamination / particles in any of them, either time. This made no difference in the problem whatsoever.

As mentioned, several mechanics ave been through the underside checking anything that could be an issue, entire drive-line has been checked, no culprits found there either.

The only thing I haven't tried is opening up the rear diff to see if there's a problem, but in the absence of 'bits' in the oil when drained (twice) I don't see what in there could be an issue.

Under no conditions other than the above is there any noise / issue. Not under heavy acceleration, towing a trailer, coasting, nothing!!

At this point I'm desperately hoping one of you have some good ideas, I'm nearly willing to try an exorcism!!


Thanks in advance.

 

Had a similar condition with my S55 AMG.. was the exhaust system. I know completely different vehicles. Question. Is it in every gear at the same rpm range or just at a range of speeds for that rpm in say top gear? Because if it is in every gear independent of speed at the same rpm then not sure it’s drivetrain because obviously the drivetrain including both diffs and everything inbetween rpm’s just differ with speed not engine rpm…the only the only parts that can change rpm independent of speed is the engine and inputs to the Tranny.

 

How are your motor mounts?
The point about it being engine rpm related and not speed related says it isn't the driveline...

 

In addition to engine and trans mounts, given what you've stated regarding engine rpm, have you changed the exhaust in anyway? Could be harmonics vibrating at a certain rpm level?

Given that it only reproduces the problem at certain rpms, and whatever load you're applying doesn't impact it, maybe its time to get the stethoscope out and get under the car and listen for variations. Might be something as simple as an exhaust hanger needing some attention. If its not obvious on first check, I'd start at the exhaust manifolds and work your way back to the exhaust tips.