Well at first guess I would say it may be the start of a wheel bearing failure.
I say that just because it's common and I've been there twice. (just replaced the rear driver side, replaced the passengar side 1.5 year ago)
One day I just started noticing the sound at over 40mph, and the faster I went, the louder it got. And the longer I waited (a couple months in each case) the louder it got at each speed range. Until I couldn't stand to drive on the freeway anymore, that's when I knew it was a bearing.
Sounds like you are in the pretty early stages if that is the case. If it keeps getting louder and louder but drives find I'd say this is it. There's probably a sure fire way to find out asap though. I heard get the car on a lift and try moving the wheel from side to side and see if it moves. I'm sure someone will chime in more on that method.
Don't rule out rear diff. though. Just offering my experiences, I'm definitley no tech.