the_pony_express

I recently started having an unusual noise coming from the rear of my car. It sounds like a whine that is correlated to my speed, no matter if I am accelerating or not. I can start hearing it as little as 5-10 mph.

I had gears installed two years ago, and I have minuscule gear whine at certain speeds, so I know what that sounds like. This sounds similar but slightly higher pitched and constant to my speed. It's not loud at all, I didn't really notice it until one time I had to turn my music down to answer my phone, but its still audible and slightly annoying. Any ideas what this could be?