Akhafeez

Hi guys, I bought a 2009 c300 used with 69k miles on it. I have been dealing with a humming coming from the rear of the car so a few weeks now. I was told by a local mechanic it was the right rear bearing, changed that and still have the sound. Another mechanic said it could be the rear diff? Any way to diagnose this? Its loudest at 30-40mph then gets drained out with higher speed but defiantly still noticeable. still there when car is in neutral..

Please help!