While on a road ride on my Trek 4300 (with slicks) I noticed a noise that increased in frequency as my pedaling increased. It appeared to be coming from the rear wheel, but I can't be positive. After I got home from the ride, I turned my bike over and checked the brake alignment, and it appeared to be fine, not rubbing the rim at all. Just for good measure, I centered both front and back brakes precisely. I checked for loose spokes, but could not find any, and I pedaled and watched the wheel pass through the brakes; no touching at all. They seemed true, however when I got back on the bike for another ride, I heard the same exact noise that increased with my pedaling intensity. To say the least, this is frustrating. Does anyone know what this might be?