This sounds exactly like what I hear in my '84 W126 300SD... it's speed dependent only, engine revs not involved.

I had an experienced indy MB shop owner drive the car who said it's the front pinion bearing in the differential. He said they'll run this way for a long while if the diff. lube is kept up to proper level and the wear/noise is not really excessive.

Since mine is not too bad I've not felt it necessary to pull the axles, the ring gear, the driveshaft, etc. to replace the bearing. Obviously, if the bearing is worn, its race should be replaced also, which is why the ring gear must come out.

Just keep the gear oil up to proper level and change it at least once a year to get rid of loose metal, however small the particles.

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Respect, protect, and maintain complex systems - whether natural or of German design, to benefit from their full potential.

1982 300SD W126.120 with an OM617.950 from a W116.120
1984 300SD W126.120 with an OM617.951