I had a humming noise coming from the rear tires for about 6 months and i was able to rule out the tires when i did my usual switch from summers to winter tires. The humming noise was still present and could be heard loudest when i reached speeds above 40mph. I could still hear the noise in neutral as I slowed down with out use of the brakes. So, I was able to rule out the tires and the brakes. This also meant it wasn't the transmission because I could still hear it in neutral.

I have extended warranty so I took it to the dealer and gave them all the info I had gathered so they could troubleshoot. It turned out the problem was worn out wheel bearings. They were replaced at no charge-problem fixed!