OK...so I've narrowed the symptoms down some more on my 98 CLK 320. When I'm at idle in park or neutral my RPM is about 700-800. If I put it in gear (R,D,3,2,1) it drops close to 500-600 and I get a slight vibration along with this humming noise. If I turn the A/C on it gets a lot worse. Any ideas? I have about 90k. I recently relocated to So. Cal so I don't have my indy back in Miami anymore....any help would be appreciated.

check my engine and trans mounts. it sounds like the torque when pulling against the tranny is moving the engine enough to touch two parts together that are not supposed to touch. a weak mount would allow more movement than is designed for.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I expect most engines with EFI now do not drop rpm with added load, as the idle speed control compensates. Is this drop in rpm 'normal' for your car, or did this recently develop?

The vibration and humming at 500 rpm could be normal, but the engine is not supposed to operate at 500 rpm.

Be sure the idle speed control is working properly.

Best Regards,
Jim