1998 C230 Rear End Noise?

I can hear a little "whine" noise coming from the rear end between 20-30 mph. We bought our car used with 60 k miles and otherwise is in perfect condition.
Car had history of a rear end seal leak which was fixed. Maybe I am fixated on that barley audible sound but it is starting to bug me. I wish i could listen to another c230 to see if in fact it is normal noise for this car. Question: Does this car have known problems with the rear end? All my recent cars have been front wheel drive so haven't heard that sound. We didn't hear that noise when we bought the car but maybe just didn't notice it before. Could I just be hearing normal rear end noise? Thanks for your input..
Larry
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If the whine occurs at a certain speed (my old 300SD's is 40 mph) AND it occurs either on application of the throttle OR no throttle - coasting - is a ring and pinion gear mesh issue that can be corrected.
Any good automotive fix it book addresses this adjustment - it usually involves adding or removing a very thin shim or two so the pinion gear is deeper or less vs. the large ring gear. The fix it books will tell you the direction that the pinion needs to be moved according to when the whine occurs.
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It could also be a faulty pinionbearing due to low oillevel but it can run for years, change when sound is not bearable anymore.

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You can try changing the rear end lube to a synthetic one... hopefully it will disappear.
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