I had identical noise from my 83 300CD. Repllacing the pinion bering is a difficult job requiring a case spreader. I could not do it nor could I find a local independent that had the tool. My local Benz dealer did not either, or maybe used it as an excuse to not work on the old car.

My solution was to replace the differential with a known good used one. Silence rules! Anyone with a spreader want to buy the old one?

BTW, that noise is non-critical so long as it does not get very loud over time or louder suddenly. Mine had a sort of soft hiss when unloaded and I went about 30k miles before the repair.

Indeed a classic sound caused by =>

At the risk of being pedantic - the spreader tool is used to set the pre-load on the axial bearings by getting the casing to flex outwards and then clamp up against the (axial) bearings. With correct shimming the crown wheel should be in such a position that the backlash between the crown wheel and the pinion wheel is correct.

Whining / droning / honing sounds are caused by poor contact between the crown wheel and the pinion wheel. In the event that the clearance of bearings on the pinion wheel (set orthogonal to the axial or side bearings - in a longitudinal sense) is too great then the pinion shaft moves as described above. The contact patch position changes and you get the sound.

In my somewhat limited experience you can dismantle the differential and crown wheel assembly quite easily from the casing without the spreader tool. Replacing the pinion bearings is then possible with the help of a hydraulic press; setting up a correct crown wheel to pinion wheel contact patch is the tricky thing. I'm in the process of doing this at the moment on a W201 differential.

The W201 differential refresh thread

This is, however, yet another project on the go - I'm going to have to make my own spreader tool to get the axial bearing pre-load correct...

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!