When I bought my car, the differential was whining loud enough to be annoying. I got it cheap because of it. That was 115K miles ago! I just make sure that it is full of lube and ignore the noise. My German immigrant mechanic, who used to work for Mercedes in Germany, told me that it would whine for a long time before it broke. He actually told me that many of them started doing it when they were nearly new.

Hope this helps.

__________________
Dave

1982 300D
352,000 miles