What causes abnormal noise in car engines?

1. The tappet will jump during operation due to excessive cam wear; 2. The valve rod end and adjustment screw or rocker arm are worn or improperly adjusted, causing the valve clearance to be too large, causing the rocker arm head of the overhead valve to be in contact with the rocker arm. Impact on the top of the valve; 3. Excessive carbon deposits in the valve guide; 4. The valve spring seat falls off; 5. The hydraulic tappet or tappet is damaged.