My car did that too, when in reverse and sometimes while sitting at a light, it was never a constant thing, and it would just happen. I have a 96 SL320, it turned out to be a bad purge valve, the idle control module would try to compensate for the vacuum loss and to do so it raised the idle. It was a messy gremlin to find and the CEL light would come on and go off, strange almost haunting, but vaccuum loss can effect idle and other things like when that darn electric pump kicks in that sounds like a vaccuum pump when cold, or I put my soft top down, this increases drag on the alternator and I can hear the idle go up.
Thanks,
Brian

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Brian W. Heitman

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