What Are the Causes of Abnormal Noises in a Car Engine?

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Engine abnormal noises can be caused by excessive clearance between the piston and cylinder wall, excessive valve clearance, or excessive clearance between the piston and cylinder wall. Excessive clearance between the piston and cylinder wall: Excessive tightness in piston pin assembly or connecting rod bearing assembly; excessive clearance between the tappet and its guide hole; wear on the camshaft lobe profile; sometimes, a loose starter clutch causing pulley noise (noticeable during speed changes). Excessive valve clearance: Excessive clearance between the tappet and its guide hole; wear on the camshaft lobe profile; excessive clearance between the valve stem and its guide; broken valve spring; radial fracture of the camshaft timing gear; loose valve seat; valve sticking and failing to close. Excessive clearance between the piston and cylinder wall: Inaccurate depth and width of the main bearing oil groove in the piston; poor lubrication due to low oil pressure.