Mystery Front End Noise - PeachParts Mercedes-Benz Forum

Dex · #1 08-06-2007, 03:10 PM

If you enjoy a good mystery, have I the noise for you. As I accelerate my 1985 300SD normally through 45mph I hear a short, not terribly loud, squeal from the front end. I can hear a similar noise on both sides of the vehicle, though . The noise might occur again around 50 mph, then always again at 55mph. If I accelerate faster than normal the noise does not occur until I reach cruising speed an let of the gas a little. Just to make things more interesting, the squeal stops when I let off the accelerator. I have also tried revving up the RPM's in my driveway and I don't hear the noise. it only occurs when I am moving.

Here's what I have done so far, all to no effect (all of it was needed anyway):
New Front Rotors
New Front Bearings
Rebuilt Front Calipers
New Brake Pads
Flushed the brake lines
New tie rods
New Steering dampener

Anyone brave enough to try to solve this mystery should not assume I did any of the work correctly, since other than brake pads, all of it was a first time for me. The tire shop guys seem to think I installed the tie rods and steering dampners correctly though. So far I have completely avoided mechanics bills with this car and you guys are my last hope to keep that record alive.

pwogaman · #2 08-06-2007, 03:48 PM

Sounds like it could be the planetary gear bushings in your transmission. Drive around and see it the sound has a correlation to when your car changes gears or you increase or decrease torgue. The planetary gears in the 722.303 are slightly helical and when the engine is torquing the gears are pushing and the force goes one way against each side bushing after an amount of wear. When you change gears or reduce the throttle the planetary gears are no longer pushing and the force is now reversed and the force is coming from the ring gear onto the planetaries - causing them to slide to each bushing on the other side - such as when "riding" the gears coming down a hill. As the planetary makes contact with the opposing bushing it momentarily squeeks.

When trying to replicate the noise, speed up to the point where you hear it and then take your foot off the pedal until you drop a few miles an hour and then speed up again. Also try it going up, and then down, an incline so you will have more force on the gears.

Attached is a generic picture of such gears.