Main Bearing Noise

The noise caused by a main bearing with excessive clearance is a loud dull knock heard when the engine is under a load. The knock is heard regularly every other revolution. If all of the main bearings have excessive clearance, a loud clatter will be heard. The noise is the loudest when the engine is lugging or under heavy load. The knock is duller than a connecting rod noise. Low oil pressure can also accompany this condition. Look for an oil light on instrument cluster. If the bearing is not loose enough to produce a knock by itself, the bearing can knock if the oil is too thin, or if there is no oil at the bearing. An irregular noise can indicate worn crankshaft thrust bearings. An intermittent sharp knock indicates excessive crankshaft end clearance. Repeated clutch disengagements can cause a change in the noise.