This might be a stuipd question, so here goes.....I've recently noticed some pretty bad humming coming from what I assume is my tires. It's from about 40mph and up, it gets louder the faster I go. I have the factory Dunlop's w/ about 400 miles on them. Tire pressures are normal, and as far as I can tell the wheel bearings are good. Is this out of the ordinary, or is this something I just need to, for lack of a better word, ignore?? Thanx in advance for the help.