Diagnosing Engine Noises

Just as we use our sense of sight to identify faults and diagnose issues with a vehicle, our sense of hearing can also be valuable in diagnosing problems.  Apart from the obvious sound that the engine produces, hearing unusual noise from the engine is a matter of concern. If you happen to hear uncommon or unusual engine noises, it requires immediate attention. Whether you like to tinker with your vehicle or prefer a mechanic’s expertise, it's important to be familiar with the most common engine sounds and their explanations. 

The following list presents the noises in order of priority. Since sounds can be interpreted differently by every individual, it is crucial to eliminate each sound in sequential order before proceeding with further diagnosis. 

A high-pitched squealing noise is commonly caused by an overly tight, loose or worn out engine belt. Depending on your engine, you may have a single serpentine belt that snakes its way around all the accessories, or you may have multiple belts that drive individual engine components. I In either of the cases, locate the specific belt responsible for the noise and check for slack and belt condition. If it's too tight or loose, adjust the belt tensioner accordingly. Replace any worn drive belts to effecitvely eliminate the sound.

Grinding noises are usually caused by worn bearings in the accessory pulleys. A bad bearing can cause the pulley to rattle. To check if the bearings on the pulleys are worn, switch off the engine and remove the drive belts. Now check each accessory and idler pulley for play and any noise as you spin it. If you hear excessive noise while spinning a pulley, you have likely identified the cause of the noise. Replace the worn pulley to resolve the issue. 

A tapping or clicking noise often originates from the cylinder head or the valve train area. To confirm the noise is coming from the cylinder head, listen to the engine and count the frequency of the noise. You can tell the difference by how rapid the noise is. If it occurs twice per cylinder revolution then it's unlikely to be from the valvetrain. A valvetrain noise will occur once per cylinder revolution. With the engine idling, use a mechanic’s stethoscope or a large screwdriver to pinpoint the exact location of the noise by placing the tip of the tool on the valve cover. Be cautious of any moving engine components. Possible causes of these noises include low oil pressure, a worn cam profile, weak valve springs, unadjusted valves, valve floating, bad lifters, damaged rocker arms, or dirt and contamination in the valve-spring assembly.

If you hear a puffing or hissing sound from the valvetrain, then the possible cause can be a broken valve. This can occur when the intense heat of combustion causes a portion of the valve's face to break, resulting in the leakage of combustion pressure through the valve. If the sound is not coming from the cylinder head, then you may have a leak in one of the vacuum hoses, intake manifold, exhaust manifold or manifold gaskets. Check where the hiss is coming from and spray some soapy fluid on that hose, gasket or manifold. Then start the engine and observe if any bubbles appear at the leak. If the bubbles are present, it indicates a leak and you must replace the affected hose, gasket or the intake/exhaust manifold. Tiny pinholes in the exhaust manifold can cause leaks which is very dangerous as it allows untreated exhaust gasses to pass directly into the engine bay and cabin. Click on the link to get more information on how to diagnose exhaust manifold leaks. 

Engine Knock is also known as detonation or pre-ignition. It occurs when combustion takes place abruptly within a cylinder before or after spark ignition and does not follow the flame front. Knocking can be caused by delayed ignition timing, excess carbon buildup, incorrect fuel octane, and faulty spark plugs. Engine knock sounds like pinging or repeated knocking noise. Modern engines have a knock sensor between cylinders that senses the knock based on which the ECU makes adjustments to the ignition timing and air/fuel ratio to rectify it. If you have an older engine without a knock sensor or if the knock sensor is broken then you may hear a knocking or pinging noise, especially at high engine temperatures. Using the correct octane fuel and swapping out bad spark plugs can fix the knock. However, if the sounds persist, then get it checked by a professional. 

A dull, heavy knocking sound from the engine block can indicate issues with the crankshaft, main bearing, rod bearings, con rod, pistons, etc. Worn main bearing noise can get very loud under heavy load. Worn out components within the engine block create a similar metallic knocking or pounding noise. Get your engine checked by a professional to determine the root cause. 

A rapid knocking or ringing noise during cold starts can be a cause of ‘Piston Slap’. This happens when the piston slams against the cylinder walls repeatedly. This can be a result of excessive cylinder wall clearance, low oil or damaged piston. If this sound goes away after the engine reaches operating temperature, then it is  likely that you have low engine oil or oil pressure. However, if the sound persists at higher temperatures, then get it checked by your mechanic immediately. 

Piston slap refers to the knocking or slapping sound that occurs when the piston moves within the cylinder and makes contact with the cylinder walls. It is a common engine noise that often occurs during cold starts and tends to diminish as the engine warms up. Piston slap can be caused by various factors, such as excessive cylinder wall clearance, worn piston rings, or low oil levels. If the sound disappears once the engine reaches its operating temperature, it is likely a result of temporary conditions like low oil pressure. However, if the noise persists even at higher temperatures, it may indicate a more serious issue, such as damaged pistons or insufficient lubrication. It is important to have a professional mechanic diagnose the problem to determine the exact cause of the problem and recommend appropriate solutions, which may involve replacing worn parts or addressing oil-related issues. 

It is never good to hear strange noises from your vehicle, especially from the engine as it most likely means expensive repairs. To prevent a light tick from turning into a loud bang you need to be aware of any unusual sound and where it is coming from to take necessary action. Do not ignore unusual noises from any part of the vehicle, because apart from the engine, you can hear unusual noises from other vehicle components too.