What is the reason for hearing grunting when idling?

The abnormal sound of purring occurs when the vehicle is idling, and the reasons are complicated, involving throttle, starting claw and so on. When the engine is idling, the mixture of gasoline and air can not be completely burned in the combustion chamber, resulting in abnormal noise in the exhaust pipe. In addition, the vehicle idling for a long time leads to too much carbon and dirt on the throttle and air intake of the engine, and the area of the air intake passage becomes smaller, which leads to the inability of the driving computer control unit to accurately control the air intake, the concentration of the mixer is too high or too low, the engine vibrates due to abnormal combustion, and the internal parts of the engine, such as the starting claw, the semi-circular key, the crankshaft pulley, the washer and the motor belt, collide with each other, resulting in abnormal noise. Million car purchase subsidy