996.2 automatic transmission signle clunking sound when start moving

[alex222]

I got a single (sometimes 2 in a row) clunking sound on a MB 996.2 automatic transmission when the car starts moving from a complete stop. The sound comes after the car moved 2-3 feet forward (or in Reverse too). I changed the ATF oil 6 months ago topping it off on a running engine then added more after switching in-gears for 10sec for each. I a little bit overflown it as now on a few days @75F a few drops came out from the breather cap in the front of the transmission a month ago and then stopped and there are a few oil drops came down in a rear on the top of the torque converter drain plug (the MB transmission is having so). The car shifts absolutely perfectly otherwise. 

 

The clunking sounds started coming a week ago when weather got some noticeably warmer and it comes not after every full stop but often. Sometimes one, sometimes two clunks. Is it because of a possible oil overfill? 

 

What this clunking sound could be after the car starts moving? With a radio on it's hardly noticeable.

 

The clunking sound is appearing coming from the RL wheel housing. I checked it's not coming from the pads nor the RL brake rotor is turning freely with a transmission in Neutral. No any scrubbing on the park brake pads which I adjusted exactly a year ago by the 996 workshop manual procedure.

Edited April 10 by alex222

[JFP in PA]

I'd start by looking at the axel CV joints on the offending corner, as well as the hub bearing.

[alex222]

Inner and outer CV boots look great on both sides. I jacked the RL wheel up and rotated it back and forth on Neutral. It has some gap when moving it back and forth but it's not making that clunking sound if rotated up in the air by hands. I jacked up and did same back and forth wheel rotation on 987 which is with ZF19 gearbox. It has similar but smaller and less noticeable gap when moving it back and forth.

 

The weel hub bearing? The wheel hub bearing on 996 seems perfect. The wheel itself rotates with no any noise.

[JFP in PA]

Try grabbing the axle near the CV joint and moving it up and down.

[alex222]

Will do on the upcoming weekend but to my knowledge as I had replaced both inner CV boots on a 355 last Summer which was involving disassembly the inner CV joints with the ball bearing completely with reassembling; if it will be a wear in the CV caves on either shaft or the star and the CV joint is loose, it will be a push because of so at the beginning of the car movement from standing still but when clunk(s) happen are when the pup is few seconds already moving and during the clunking sound there is no any pushes or any movement distortion. As I looked the 996.2 and 987.1 final drives and the CV boots look very similar to 355. Probably the same or similar all made by Bosch.

 

It is just the clunking sound. Or two. And, when driving somewhere say for about an hr, the sound is not getting repeating more often, etc. And I was driving at some times up to  60mph. All perfect. Shifting and everything.

[JFP in PA]

Then I would also be looking at the Allan head fasteners that hold the assembly together; we have seen MANY of these back out because they were not cleaned and torqued properly with Loctite applied to them before they were reinstalled.  Very common post work failure point. 

 

[alex222]

I lifted up the pup in a body shop I had stopped by for something else and checked the final drives and CV joints both tight and the inner joints are tight to the gearbox flanges. Seems all good. Not seeing anything loose in rear suspension on neither side. 

 

I have a spare set of aftermarket rear brake rotors with the aftermarket set of the pads in my garage that I replaced with the factory ones a year ago after I got the pup. Will swap those one side at a time see if it anything w/ the stock brake disks/pads or also there is an aluminum screen on the inner side of the brake rotor if anything is with it?

[alex222]

It was a cross-bar under the transmission pan got loose because I had overfilled a bit so the oil got to the mounting bolts + vibration.