pit Bike?

if its got a disc brake could be the pads moving in the caliper or wheel bearings shot. we'll assume there's nothing stuck to the tyre that's hitting the mudguard when the wheel spins. can't do much about rattling pads but if you take the front wheel out and turn the wheel spindle, if the bearings have gone the spindle may not turn easily or you will feel the roughness or even hear it when the spindle turns.

Jimmy
March 2012