My W115 differential became noisy, and so I ordered an as-is differential with a different casing (but same gear ratio) on eBay for a couple hundred bucks, and a shop was able to swap out the pinion - and the noise went away. They found some weird pitting / wear pattern on the original pinion. Didn't cost me too much to have this done.

-Henry

__________________
1976 Mercedes-Benz 300D, Colorado Beige / Mahogany
www.hmclever.com