I have a 91 300E 4Matic with a LSD rear diff and 130K miles. I have been getting a mild winning from the rear diff. and from reading the past post many believe that the pinion bearing would be the noise maker. I had my car in to a MB dealer and had them listen/test drive to get an opinion on the pinion.

They said they believe that it is the noise maker and the mechanic believes I can drive for years with out a failure with it making that noise. The dealer cost to replace the LSD rear Diff. is $3000.00. I think that might just be the part?
I will drive it as is.. I have drained and refilled with Mobil1 Synthetic 75W90..

OK I read more searching under Pinion Bearing. Should go a long time and Mobil1 75w140 or red line 80W140 synthetic maybe used in an attempt to reduce noise.

Still if you have a different........... Please post if you have any experience or opinions on how long one may drive a MB 300E that is making this noise, assuming it is the pinion bearing.. Also if the noise will just get worst over time.