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

I would start with a trans fluid & filter service, use a good quality synthetic Dexron ATF

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The 722 repair manual actually references a light grinding or "rolling" noise in P or N, which is the front planetary gear set. It is considered normal to some extent.
It goes on to say if in doubt compare to a similar vehicle.

A similar noise in Reverse is attributed to excessive release clearance in the B3 clutch disk pack. This could be due to worn B-3 clutch disks.

In my experience, the rolling noise in neutral/park is normal; mine has noticeably done it for the last 100k miles at least - it now has about 230k.
It's not excessively loud, and cannot be heard if the heat or air is on low or a window is down, allowing other engine noise in.

Also, it seems on the 722 trans that the first thing that fails is the b-3 clutch pack - the fiber plates wear excessively, and eventually come apart. The first symptoms are a delay when shifting to reverse. Later, a clicking is heard when moving in Reverse, as the clutch basket moves so far back due to the worn plates that it strikes the housing. Time to get it repaired quick when that happens. Mine went at about 150k. Since I do my own repairs, i opted to pull the tranny and replace only the worn clutch plates; a pretty easy job as they are right at the front. Everything has been good for 80k since.

You mentioned there is no delay when shifting to R, so that might indicate no real problem with the b-3 plates yet.

As stated in the previous post, you should drain the fluid, pull the pan and look for debris or signs of problems. If nothing unusual is found, pour in new fluid and don't worry about it.

DG

Great info DG...thanks! The transmission fluid and filter were changed only 3 weeks ago. It was done shortly before I got the car so I don't know how the old juice looked. Looks like clear red syrup now!

Not so bad going in to reverse, but engaging Drive comes with a bit of a clunk. I've had two mechanics look at it and say..."yes, I hear it and agree it's probably not as it should be, but not bad either. Drive it and enjoy". So that's what I'll do! The car only has 68k on it which is why any issue at all is baffling. Perhaps I'm being too picky!

Thanks again for the feedback guys. Much appreciated.

Mike, if the clunk sounds like it might be due to the driveline torquing over toward the driver side when shifting to D; you might try the following:
- Double-check the motor and tranny mounts for correct type and mounting - you never know.
- on my car the exhaust pipe can bang against as cross member brace when the driver-side mount sags a bit. Any close-fitting exhaust pipe could be the culprit.
- If yours has the little shock absorbers mounted next to the motor mounts, these don't have much travel, and can actually bottom out if there is anything amiss with the motor mount height, like sag, or the wrong mount.
makes a great clunk when shifting to D - ask me how I know.

Get someone to shift the car from R to D a few times while you watch and listen under the hood. Drive the car and punch it in low gear and listen for the clunk as the engine torques over to the right.

If it's really coming from a delay in the tranny, then not much you can do externally.

DG