I have an 18 speed road bike and whenever I pedal, I hear a clicking noise every time I rotate past the top of the pedal cycle. It sounds exactly like what you hear if a gear doesn't shift properly, only that's not the problem. This happens on every speed when I'm riding, however it doesn't make this sound if I were to just turn the bike over and rotate the pedals with no weight or wheel movement on it. What do you think could be causing it?