There are four steps to diagnosing engine noise. The technician must determine the following conditions:

Remember, engine noises are generally synchronized to either the engine speed (indicating the noise is caused by the crankshaft, connecting rods or pistons) or one-half engine speed (indicating the cause is in the valve train).

Damaged or worn main bearing noise is revealed by dull thuds or knocks which happen on every engine revolution. Bearing noise is loudest when the engine is under heavy load.

Excessive crankshaft end ply is indicated by an intermittent rap or knock that is sharper than the sound of a worn main bearing. Causes of a main bearing noise include the following:

A damaged or worn connecting rod bearing will produce a knock under all speeds. During the early stages of wear, connecting rod noise may be confused with a piston slap or loose wrist pins. Connecting rod knock noise increases in volume with engine speed and is at its loudest on deceleration. Causes of connecting rod bearing noise include the following:

Engines designed with timing belts and sprockets can produce different noises. The most common noise is a high-frequency, light knocking sound. This sound will generally be the same in intensity, whether the engine is idling, operating at high speeds or under load. Causes of timing belt and sprocket noises include the following:

Piston pin, piston and connecting rod noises are hard to separate. A loose pin causes a sharp double knock usually heard when the engine is idling, or during sudden acceleration then deceleration of the engine. A piston pin that has been improperly fitted will not load on the engine.

Excessive piston-to-cylinder bore clearance will cause a piston slap noise. The noise is similar to a metallic knock, as if the piston were slapping the cylinder wall during its stroke. As with most engine noises, understanding the cause of the noise will help you imagine what the noise sounds like. An indication of piston slap is a decrease in noise as the engine warms up. When the engine is cold, the piston-to-cylinder bore clearance is greater and piston slap will be louder. Causes of piston noises include the following:

A light tapping at one-half engine speed, or any varying frequency, can indicate a valve train problem. These tapping noises increase with engine speed.

Before attempting to evaluate valve train noises, thoroughly warm up the engine. This will bring all engine components to a normal state of expansion. Also, run the engine at various speeds and listen for an engine noise with the hood closed while sitting in the driver's seat.

The causes for valve noises include the following:

Notice: Be sure the converter-to-flywheel bolts are not too long. Converter bolts that are too long may dimple the torque converter clutch apply surface causing a shudder condition.

A loose or cracked flywheel will produce an irregular thud or click. To test for a loose or cracked flywheel, operate the vehicle at approximately 32 km/h (20 mph) and shut off the engine. If a thud is heard, the flywheel may be loose or damaged. This type of thud is loudest upon deceleration.

A loose torque converter-to-flywheel or flywheel-to-crankshaft bolts will sound similar to bearing knock. These conditions produce several raps during quick acceleration on a free running engine. Depending on the idle smoothness when the transaxle is in gear, the noise may or may not appear. Check the torque converter-to-flywheel and flywheel to crankshaft bolts before attempting to investigate any bearing related knock.