Fault phenomenon: a 2010 FAW Volkswagen Sagitar car has a cumulative mileage of about 70 thousand km, and the engine will produce a "buzz" noise when working. Inspection and analysis: the inspection found that the engine humming when it was working, no matter whether the engine was in low speed or high speed. The engine was monitored with a diagnostic device. It was found that the noise was in the front of the engine, but it was difficult to judge the specific location of the sound source, so the external transmission belt of the engine front end was dismantled and the difference was eliminated one by one. After removing the transmission belt, the sound did not disappear, so the part of the transmission assembly was eliminated. Open the hood of the engine, it is found that there is a wear mark on the outer edge of the timing belt, the drive belt links up, and the hood is carefully checked, and the inside is worn out, so it is inferred that the "buzzing" sound should be caused by the friction between the positive time belt and the normal time belt cover. After starting the engine test, the noise disappeared when the timing cover was removed, but it was found that the timing belt moved outward. It is concluded that the "buzzing" sound generated by the engine should be caused by the eccentric wear of the timing belt when the timing belt moves outward. When the positive time belt is replaced, the abnormal sound of the test "buzzing" appears again. It is doubted that the timing belt may be caused by the positive time belt outward movement.