Type of sound	Condition	Possible cause
	Regular clicking sound	Sound increases as engine speed increases.	• Valve mechanism is defective. • Incorrect valve clearance • Worn valve rocker • Worn camshaft • Broken valve spring
	Heavy and dull clank	Oil pressure is low.	• Worn crankshaft main bearing • Worn connecting rod bearing (big end)
		Oil pressure is normal.	• Loose flywheel mounting bolts • Damaged engine mounting
	High-pitched clank (Spark knock)	Sound is noticeable when accelerating with an overload.	• Ignition timing advanced • Accumulation of carbon inside combustion chamber • Wrong spark plug • Improper gasoline
	Clank when engine speed is medium (1,000 to 2,000 rpm).	Sound is reduced when fuel injector connector of noisy cyl­inder is disconnected. (NOTE*)	• Worn crankshaft main bearing • Worn bearing at crankshaft end of connecting rod
	Knocking sound when engine is operating under idling speed and engine is warm	Sound is reduced when fuel injector connector of noisy cyl­inder is disconnected. (NOTE*)	• Worn cylinder liner and piston ring • Broken or stuck piston ring • Worn piston pin and hole at piston end of connecting rod
		Sound is not reduced if each fuel injector connector is dis­connected in turn. (NOTE*)	• Unusually worn valve lifter • Worn cam gear • Worn camshaft journal bore in crankcase
	Squeaky sound	—	• Insufficient generator lubrication
	Rubbing sound	—	• Defective generator brush and rotor contact
	Gear scream when starting engine	—	• Defective ignition starter switch • Worn gear and starter pinion
	Sound like polishing glass with a dry cloth	—	• Loose drive belt • Defective water pump shaft
	Hissing sound	—	• Loss of compression • Air leakage in air intake system, hoses, connections or manifolds
	Timing belt noise	—	• Loose timing belt • Belt contacting case/adjacent part
	Valve tappet noise	—	• Incorrect valve clearance