Tech Tip: Becoming Attuned to Engine Sounds – UnderhoodService

Tech Tip: Becoming Attuned to Engine Sounds

Can you pinpoint abnormal noises coming from an engine? More often than not, internal engine malfunctions will reveal themselves first as an unusual noise. This can often happen before the problem affects the driveability of the vehicle. Unless a technician has experience in listening to and interpreting engine noises, it may be hard to distinguish one from another ....

Can you pinpoint abnormal noises coming from an engine? More often than not, internal engine malfunctions will reveal themselves first as an unusual noise. This can often happen before the problem affects the driveability of the vehicle.

Unless a technician has experience in listening to and interpreting engine noises, it may be hard to distinguish one from another.

When correctly interpreted, engine noise can be a very valuable diagnostic tool. For one thing, a costly and time-consuming engine teardown might be avoided for your customer.

Careful noise diagnostics also reduce the chance of ruining the engine by continuing to operate the vehicle despite a growing problem.

Some engine sounds can be easily heard without using a listening device, while others may be impossible to detect unless amplified. Stethoscopes help you locate the source of engine noise quickly. A trained technician will listen for trouble by tracing the sound of flowing water, oil, gas or steam, allowing him or her to identify piston slap, worn gears, faulty valves, water pump failure, damaged gaskets and/or defective bearings.

The procedure using a stethoscope is fairly simple — use the metal prod to trace the sound until it reaches its maximum intensity. Once the precise spot is determined, the sound can be better evaluated.

Better results may be obtained with an electronic listening device, which allows the user to tune into the noise.

Stethoscopes and electronic listening devices can be obtained from your mobile tool distributor or from most retailers, jobbers and tool and equipment suppliers.

Caution: Since abnormal engine noises must be detected when the engine is running, be careful when listening for noises around moving belts and pulleys. Also, keep the stethoscope or electronic probe away from moving parts. You could be injured if the stethoscope or hose is pulled inward or flung out by these moving parts.


Main or Thrust Bearing Noise: A loose crankshaft main bearing produces a dull, steady knock, while a loose crankshaft thrust bearing produces a heavy thump at irregular intervals. The thrust bearing noise might only be audible on very hard acceleration.

The Fix: To correct this problem, replace the bearings or crankshaft.

Piston Pin Knock: This is a sharp, metallic rap that can sound more like a rattle if all the pins are loose. It occurs in the upper portion of the engine and is most notable when the engine is idling and is hot. Piston pin knock sounds like a double knock at idle speeds. It’s caused by a worn piston pin, piston pin boss, piston pin bushing or lack of lubrication.

The Fix: To correct this problem, either install oversized pins, replace the boss or bushings, or replace the piston.

Piston Slap: This sound is commonly heard when the engine is cold and often gets louder when the vehicle accelerates. When a piston slaps against the cylinder wall, the result is a hollow, bell-like sound. Piston slap is caused by worn pistons or cylinders, collapsed piston skirts, misaligned connecting rods, excessive piston-to-cylinder wall clearance or lack of lubrication, resulting in worn bearings.

The Fix: Replacing the pistons, reboring the cylinder, replacing or realigning the rods or replacing the bearings will correct this problem.

Ring Noise: This sound can be heard during acceleration as a high-pitched rattling or clicking in the upper part of the cylinder. It is usually caused by worn rings or cylinders, broken piston ring lands, or insufficient ring tension against the cylinder walls.

The Fix: Ring noise is corrected by replacing the rings, pistons or sleeves, or reboring the cylinders.

Ridge Noise: This sound is less common, but very distinct. As a piston strikes the ridge at the top of the cylinder, the result is a high-pitched rapping or clicking noise that becomes louder upon acceleration.

The Fix: There may be more than one reason for the ridge interfering with the ring’s travel. If new rings were installed without removing the old ridge, the new rings will contact the ridge and make the noise. Or, if the piston pin is very loose or the connecting rod has a loose or burned out bearing, the piston will go high enough in the cylinder for the top ring to contact the ridge. In order to eliminate ridge noise, remove the old ring ridge and replace the piston or piston pin.

Rod-Bearing Noise: The result of loose or worn connecting rod bearings will cause a noise at idle, as well as at speeds above 35 mph. Depending on how worn the bearings, the noise can range from a light tap to a heavy knock or pound. Rod bearing noise is caused by a worn bearing or crankpin, a misaligned rod, or lack of lubrication.

The Fix: Service or replace the crankshaft, realign or replace the connecting rods and replace the bearings.

Rumble & Thump Noises: A loose vibration damper will cause a heavy rumble or thump in the front of the engine that is more apparent when the vehicle is accelerating from idle under load or is idling unevenly. A loose flywheel causes a heavy thump or light knock at the rear of the engine, depending on the amount of play and the type of engine.

The Fix: Both of these problems are corrected either by tightening or replacing the damper or flywheel.

Tappet Noise: This noise is characterized by a light, regular clicking sound that is more noticeable when the engine is idling. It is the result of too much clearance in the valve train. The clearance problem area is located by inserting a feeler gauge between each lifter and valve, or between each rocker arm and valve tip, until the nose subsides. Tappet noise can be caused by improper valve adjustment, worn or damaged parts, dirty hydraulic lifters or lack of lubrication.

The Fix: To correct the noise, adjust the valves, replace any worn or damaged parts, or clean or replace lifters.

Source: Automotive Technology — A Systems Approach, by Jack Erjavec and courtesy of Delmar Cengage Learning —

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