Engine noises are usually sensitive to changes in rpm and load.
For example, a collapsed lifter will make an audible “ticking” noise that increases in frequency as rpm goes up.
Engine noises can also be determined according to speed of the related rotating component. For instance, valvetrain noises will be at a frequency that is one half crankshaft speed.

Noise caused by engine accessories such as the vacuum pump alternator, power steering pump, A/C clutch, or drive pulleys is speed sensitive. An increase or decrease in engine rpm will generally change pitch and frequency, or even cause it to stop.
A mechanics stethoscope will generally help pinpoint a noise source. Removing the accessory drivebelt will also help verify an accessory generated noise.

Bearing noise can be differentiated by pitch. A damaged connecting rod bearing makes a higher pitched, metallic knocking sound. This as opposed to the lower pitch thump of a worn, or
spun main bearing. A failed rod bearing can be confusing as it seems to make a greater frequency noise. This is because the bearing may knock at both ends of piston travel.

Piston and Connecting Rod
Piston slap is usually caused by severely worn cylinder bores and pistons, partially collapsed piston skirts, worn pin bores, severely worn rings, or an undersized piston. An incorrectly assembled, or installed connecting rod and piston assembly will also produce slap.
Slap always occurs at crankshaft speeds. In severe cases, slap may occur in both directions of piston travel. Although fairly common on high mileage engines, slap can be difficult to hear on low mileage engines. As the term implies, piston slap is appropriate for the sound generated. It occurs when a piston begins to rock within the cylinder as it travels up and down. This action causes the skirt to slap the cylinder wall as it straightens, then rocks away from the direction of thrust.

Valvetrain and Camshaft
Rocker arm and hydraulic lifter noise is probably the most easily identifiable. Both make tapping (or clicking) noises that only differ in pitch and volume. Although damaged valve springs or pushrods are more difficult to hear, they usually cause a power loss or rough engine condition and require inspection to locate.
A worn, missing, or incorrect thickness camshaft thrust washer will cause excessive cam end play. The most frequent result is an audible knocking sound localized at the rear of the block.
Timing gear and chain noise is not always audible, even when the chain and gears are severely worn. These components, when failed, are usually discovered during timing checks prompted by a low power or no-start condition.
NOTE: Valvetrain noise can also be the result (or first indicator) of low oil pressure.

Starter Noise
The starter bendix can hang and prevent quick disengagement after startup. The resulting noise can be misdiagnosed as engine related. A mechanics stethoscope is fairly effective at differentiating engine and starter noise.

Knocking Noise At Idle
Knocking noises can be from connecting rod bearings, a cracked flywheel, converter touching the housing or cover, A/C compressor, or loose exhaust component.
Look for exhaust components grounding against the body, frame, or driveline component. Remove the converter access cover and visually inspect the flywheel and converter. Check the cover and converter housing for signs of contact. A flywheel cracked at the hub will allow the converter to wobble slightly. Test the A/C system for incorrect charge levels which can produce compressor knock.

Diesel Fuel Knock or Rattle
The rattling sound unique to diesel engine is normal. It is a function of high compression ratio, injector nozzle spray pattern and pressure, and compression ignition. However, incorrect injection pump timing, low quality fuel, or injector nozzle faults can make the normal sound much more pronounced. If the normal rattling is accompanied by, or becomes a knocking sound, the fuel injectors, injection pump, pump timing, and fuel quality must all be tested.


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