I am hearing a hum coming from the back end of my model S. The pitch does not change however it grows louder at higher speed. It also does not matter what type of road surface I drive over, but some like grooved concrete make it a lot louder. It is almost as if something is resonating. That's the only thing I can think of because the pitch never changes. I hear it at speeds as low as 40, even on a road that has just been repaved. My other guess is it might be a bearing. It is not the inverter sound and it does not change if I am accelerating, maintaining a steady speed or decelerating. I also have the 21" rims with Michelin Super Sports. Anyone else have this issue? Update: The problem is now noticeably worse. It feels like something is grinding in the drivetrain and it is louder and starts at a much lower speed now. I contacted Tesla service but they said they can't see me until 10/13. I think I may end up having to be towed in before then given how quickly things are worsening.