2001 BMW 325 At 65 MPH and up, there is a base sounding noi

Hi.

I have a noise coming from the rear end of my BMW that sounds like a base sub-woofer. This only occurs when I am at 65 to 75 mph and I am pushing the gas pedal to maintain speed.

There is no loss of power, just this deep base noise. If you excellerate, the car goes great, but when you return to the cruising speed of 65 to 75, the sound returns.

I have had the the rear end differentual checked, the drive shaft and bot rear wheel axiles checked, but nothing helps, so I am hoping someone can.

Thanks in advance