New (to me) 88 560SL. Car has 68k verified miles and in very nice condition. Car starts and runs great. But...when in neutral or park, there is a noticeable hum/whine that sounds like it's coming from the trans. When I put it in Drive, there's a slight pause then there is a "clunk" as it goes into D. It engages Reverse just fine but with a whine when backing up. Once in D it drives quietly and shifts very smoothly.

Some background: Car has recently had new engine, trans and subframe mounts. CV joints, U-joint, flex disc and diff mounts are all good. Trans fluid and filter are new. All new belts and fresh oil change. My mechanic found a vacuum leak at the transmission modulator and repaired/adjusted it, but it made little/no difference.

He wasn't able to diagnose the problem but felt it would be fine as it is without risk of catastrophic failure. But...I know it's not right, and it bugs me. Open to suggestions, ideas or similar experience.

Thanks,

Mike