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.