Lately the bike has started squeaking when I ride it, like the sound that riding a mountain bike with a buckled wheel makes, if you get me? Like a rhythmic sound, so it speeds up if I do, and slows down if I do. I can't hear it a high speed because of the engine noise, but its really loud at slow speed. Any ideas? Someone said it might be dust on the brakes, but it does it even when I'm not using the brakes.