I've had this car 4-5 months and it's not had any problems. But recently it has developed a humming sound in the rear end at highway speeds. It is getting progressively louder. It doesn't seem to correlate with RPM or brake applications but road speed only. Seems to be constant, not so much a pulsating noise like I'd expect from a wheel bearing - but I haven't had much experience in many years with failing bearings. I tried to search this board, but didn't see anything addressing this issue. Any ideas on what this is or how to diagnose it?

Thanks in advance......Calvin