What about the rear end? I had a problem with my 140 where it was making a humming noise, drove me nuts. After throwing several grand at it, I took it to a specialist at Park Place, he knew exactly what it was, it was a bearing on the rear. Sometimes when you have the weight and force off a car you can't feel the play, take a look at the transmission mount too. Also check the strut arms on the rear to the hub carrier, if you have worn bushing here that can cause funny noises. I would think that all the things that you checked are what any good MB tech would look at first, but if you can't find it, get a specialist.
Good luck!

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Brian W. Heitman

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