The noise generation mechanism of flat belts on a timing pulley, trapezoidal tooth profile timing belts and circular tooth profile timing belts has been experimentally studied. The results of this study are as follows. (1) The noise generation mechanisms of the three kinds of belts are all the same. The noise is mainly an impact sound due to the collision of the belt and the top land of the pulley tooth at the entry point of the driving pulley. (2) The impact noise is reduced by the interference of either the belt and the pulley tooth flanks or the top land of belt tooth and the bottom land of the pulley tooth groove. (3) The noise level of the trapezoidal tooth profile timing belt is high when the belt pitch, expanded by the tension, is equal to the pulley pitch.