[R129 SL] Rumbling while idling in "D"

[Baktash Taghehchian]

I've had my 1992 500SL for about 2 years now and done
lots of regular maintanance to it. Mostly done myself,
but sometimes had to take it to the dealer. Recently a
new problem has popped up and for the life of me I
can't figure out what the problem is. Here's what is
happening. When the car is in PARK, NEUTRAL, or
REVERSE it idles just fine with no abnormal noise or
vibration. When I put the car in DRIVE or any other
forward gear, the car idles a little rough. There is a
very loud humming/vibrating. But as soon as I apply a
little throttle the noise/vibration goes away.

Now, here is what I have done so far without any luck.
I replaced the sparkplugs with new Bosch platinum4
plugs, replaces the worn serpentine belt, replaced the
airfilters with new K&N filters, replaced the
fuel-filter. And the noise has not changed at all. Of
course the car runs a little better while driving, but
the idling problem is still there.

Any suggestions??

Thanks,
Baktash Taghehchian
1992 500SL

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