Sounds like a pinion bearing to me. Its common for MB differentials to be noisy for years without getting worse.
You probably just need to decide whether it is getting worse or if it is bothering you too much.
Not too many techs do differential repair now days. We usually replace with used for most MBs as they are very good gearboxes generally. This means two things. The box you get will probably last the life of the car and more importantly they don't charge much for them as they are not a big seller. Some parts they just can't get rid of out of salvage; MB differentials would definitely be one if the cars weren't used forever in some cases. We are finally having trouble getting good used differentials for 108 chassis (out of production for 28 years).