My '88 190E 2.6 axle had a noise problem when new - a high frequency "singing" under float conditions that was very annoying. Their first solution was to swap the entire axle assembly - one that had been specially processed for low noise. (I tried to get them to install a LSD version from the 16V, but the factory rep wouldn't go for it.) They admitted that some 190Es had an axle noise problem, but it had nothing to do with the fact that mine is a rare manual transmission version.
The new diff had the same noise. In conversations with the MB rep and service manager I suggested opening up the axle and altering the backlash, but they said that the axle was not considered a field serviceable item, and I don't think you can buy internal parts through the parts system.
About a year later when my car was in for more of its long saga of warranty work, the service manager told me they were going to swap the axle again. I was lead to believe that it actually had a different ring and pinion tooth count of approximately the same ratio. In any event, the this latest differential was void of the "singing" problem.
I'm glad this car had a 4 year/50K mile warranty as it was really needed. I also have to credit DB with excellent service. They never gave up on any of the myriad problems, but the best new car warranty is the one you never need to use.