Replaced diff works but with issues

After a great headache for 6 months with first humming noise and then progressing to a jarring grating or ratcheting sound emanating from the belly of my 2003 e46, the culprit was finally nailed: it was the diff! I fitted another diff (used) and now I have a great smooth ride with hardly a sound!!! But with one issue: at around 60kph, there is a sound like the car is moving through a huge wind! I cannot quite describe it but u will get the picture if u have ever experienced the sound an earthquake makes. When I reach about 80kph the sound disappears. What is the problem now? Could it be that probably the diff is not the exact fit? I cannot tell if the replaced one was the exact part number as the original? Could it be the workmanship of the mechanic in replacing the diff?

Add another possibility; the used diff has a flaw.

Sure the center bearing is good? Driveshaft was mated properly?
Diff mounting bushing in good shape?



http://www.bimmerfest.com/forums/sho...d.php?t=681143

Mechanic said the center bearing was fine but I am taking his word. I am going to get another opinion about the other items. Thanks