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.
Connecting Rod Bearing Noise
Connecting rods with excessive clearance knock at all engine speeds, and under both idle and load conditions. The knock is usually rhythmic and the speed and volume will increase and decrease relative to engine speed. When the bearings begin to become loose, the noise can sometimes be confused with piston slap or loose piston pins. Low oil pressure can also accompany this condition. Look for an oil light on instrument cluster.
Piston Noise
It can be difficult to tell the difference between a piston pin, connecting rod, or piston noise. A loose piston pin causes a loud double knock which is usually heard when the engine is idling. When the injector to this cylinder is cut out, a noticeable change will be heard in the sound of the knocking noise. On some engines the knock becomes more noticeable when the vehicle is operated on the road at steady speed condition.