Engine Noise-Where Is It Coming From?-Is It Bad?

With a little practice a do it yourself auto mechanic can gain the experience and knowledge to interpret what engine noise means.

Always take the extra time to make a proper noise analysis before doing any engine repair work.

Also having a heads up on the noise can reduce repair costs by addressing the problem before irreversible engine damage occurs.

When diagnosing engine noise, don’t forget basic techniques like running it with the drive belts removed, dropping the transmission into Drive and checking for broken accessory mounts.

Most techs are familiar with using a long screwdriver, socket extension or even a piece of hose as a stethoscope. A mechanic’s stethoscope will provide much clearer and accurate results.

Consequently, We’ve put together this quick guide to diagnosing common engine noises to help you avoid potential damage.


Engine Noise
Engine Noise

Valvetrain Noise

Valve and tappet noise usually begins as a clicking sound, or chatter, at half engine speed and may then disappear at high speeds. The cause is often excessive valve clearance or a defective hydraulic valve lifter.

Worn or sticking hydraulic lifters can cause this noise.

  • Varnish build up on the lifter surfaces.
  • Low oil pressure could be another cause.

To check your clearances, you can insert a thickness gauge between the valve stem and the rocker arm or lifter. If this reduces the noise, the cause is excessive clearance, and you’ll need to make the proper adjustments. Other things to look for include lifters that are moving loosely in their bores and weak valve springs.


Detonation, Pre-Ignition (Pinging) Noise

Detonation can cause serious damage to an engine. You usually hear this noise when accelerating the vehicle. Most people call this a pinging or rattling sound. This noise is caused by an air/fuel mixture in the cylinder being ignited prematurely. If ignition happens before the piston reaches the top of its stroke, this is called pre-ignition or pre-detonation, which can damage the pistons, valves and connecting rods. They get damaged because the fuel igniting too early produces pressure waves from the fuel’s explosion in the cylinder, which collide with the cylinder as it’s moving up. And that’s also why you hear the pinging and rattling noises.

Some of the causes of this condition are improper fuel octane, engine overheating, improper ignition timing, the EGR valve not functioning properly and problems with the computer or knock sensor. This creates multiple flame fronts in the cylinder fighting each other and causing the pinging and rattling noise. Check your owner’s manual to make sure you’re using the right grade of fuel. Or you can switch to a higher grade for a period and see if the noise goes away. If it doesn’t, you’ll want to look at these other possible causes.


Timing Chain Noises

Many of the newer engines have overhead camshafts with longer timing chains. A timing chain connects the crankshaft to the camshaft to insure the valves open at the proper time. Hydraulic tensioners usually keep the slack tight on the chains. The chains ride against a nylon guide (a chain guide) which, in time, begins to wear. At the point where the chain guides are worn beyond the ability of the hydraulic tensioner to take up the slack, the timing chain begins to rattle.

This noise is caused by the timing chains becoming so loose that they whip back and forth against the guides and possibly the timing cover. If the oil pressure is correct, replacement of the hydraulic tensioners and chain guides would be required. A mechanic’s stethoscope is a great tool to pinpoint this noise. If the noise is loudest when touching the timing cover with the stethoscope disassembly would be required to confirm and to repair the problem.


Connecting Rod Noises

Connecting rod noise is caused by excessive clearance between the crankshaft and the connecting rod bearing surface. This happens when you have low oil pressure causing the bearing to run dry of lubrication, which in turn will damage the bearing and crankshaft surfaces. This can also be caused by poor maintenance practices such as not changing oil at a regular interval. The oil gets dirty and grit can wear the surface of the bearings.

The noise is usually heard when you hold the throttle at a steady RPM. If it sounds like a single knock, you can isolate the cylinder by disabling the spark or the fuel injector for each cylinder one at a time. When the noise goes away or gets much quieter, you have found the problem. Problems like this require immediate attention because continued running of the engine in this condition will damage the crankshaft and require a major engine overhaul.


Piston Pin Noise

Although similar to valve train noise, piston pin noise often has a unique, metallic-sounding double knock and is sometimes most noticeable during idle with the spark advanced. This noise is usually caused by the lack of oil and excessive clearance between the piston pin and the piston. A piston pin attaches the connecting rod to the piston. It is lubricated by oil which is sprayed onto the pin through a hole in the opposing cylinder’s connecting rod.

As a result, This condition can only be remedied by replacing the piston pin bushings, possibly even the piston itself, along with solving the oil pressure or lubrication problem. Problems like this are usually a result of worn connecting rod and crankshaft bearings which reduces oil pressure. As with connecting rod noise, you can find the offending components by performing the same test outlined above.


Piston Ring Noises

Piston ring noise is also similar to the valve and tappet noise above; however, it is most noticeable during acceleration.

Most often, this noise is caused by,

  • Low ring tension
  • Broken or worn piston rings
  • Worn cylinder walls

To troubleshoot each cylinder, remove the spark plugs and add a tablespoon of engine oil to each cylinder.  Then, crank the engine for several revolutions to work the oil down past the rings. You can then install the spark plugs and start the engine. As a result, If the noise is reduced, the rings are most likely the problem.


Piston Slap Noise

A hollow, muffled, almost bell-like sound is usually piston slap. This condition is caused by a piston rocking back and forth in its cylinder. Continuous piston slap means the engine needs service; however, if you only notice this sound when the engine is cold, it is likely not serious.

A continuous piston slap sound is usually caused by,

  • Worn pistons
  • Excessive piston-to-wall clearance
  • Misaligned connecting rods
  • Worn cylinder walls
  • Inadequate oil

Bearing Noises ( Rod Knock )

A heavy, yet dull metallic knock is typically bearing knock. Under load or acceleration the noise is louder.

  • A regular, rumble-like knock is often from worn main bearings.
  • More distinct knock is routinely attributed to worn rod bearings.
  • A sharp, irregular knock can be from worn thrust bearings.

Remember, though, that there are multiple places in the engine where bearings are used.  As a result, there are several items under the hood that can make this kind of sound,

  • Water pump
  • Air conditioner clutch bearing
  • Fan belt idler pulleys or belt tensioner
  • Alternator and the power steering pump

So, These are all possible sources for sounds indicating future bearing failure.


Conclusion

When diagnosing engine noises, don’t forget basic techniques like running it with the drive belts removed, dropping the transmission into Drive and checking for broken accessory mounts. Remember, it’s always wiser to spend more time diagnosing and less time fixing.

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