you know, this same thing happened to me, and i was freaking out that there was something wrong with the bottom bracket, the drivetrain, something. i was about to rip the bottom bracket, check the bearings, and repack it. wisely, i took it to the local co-op first to ask for advice, and the guy took it for a quick ride around the block.

"did you hear the sound i was talking about?"

"yeah, it's your tire. it's a little crooked and is rubbing against the chainstays."

check even the obvious things.