What is the reason for the loud noise of automobile engine?

The cold car can't get enough lubrication when it starts, and the hydraulic tappet and mechanical rocker arm can't work normally, which leads to clicking sound. Incorrect oil condition: If the oil viscosity is too high or too low, the engine will produce noise. Insufficient belt tightness: the engine squeaks when working. Air leakage in the air inlet and exhaust pipe: At the air inlet and exhaust pipe, gas will leak from the exhaust pipe, resulting in noise. Poor fuel quality: it will cause serious vibration and noise of the engine. Loose plastic parts in the engine compartment: noise will be generated when the engine working room collides with the engine compartment.