Diagnosing Engine Sounds

By: Ray Bohacz

During my many years of working with machinery, there has been only a handful of times when I heard a strange noise and did not think the worst…if only even for a few seconds.

It is essential to establish that there is no “official” way to identify an unusual sound. The one protocol that needs to be followed is not to be closed-minded to the source or location. If so, you will often be confounded.

These examples come to mind from my youth when I worked as a mechanic: a rattle as if the bumper were falling off, and a high-frequency metallic sound from under the hood.

The rattle was in a doctor’s car and ended up being a pork chop that was intended to be his lunch that fell into a well alongside the fender in the trunk.

The other, a slightly loose A/C compressor belt that would send a harmonic into the compressor bracket when the clutch was engaged and the engine at about 2,000 rpm.

In both instances, the worst was presumed.

In contrast, a Toyota engine with a slight tap after a hot restart, which presented like a cam follower, was an exhaust valve seat loosening up.

It was discovered when the customer restarted the engine after it being off for only a few minutes.

The seat dropped and broke the valve. It then collided with the piston and removed it from the connecting rod. It exited through the side of the block.

Thus, Lesson #1 locating a noise is not to minimize it or believe it is catastrophic. The diagnostics require an intelligent and systematic approach.

Though it is impossible to list every cause for a strange sound, this primer will help you get to the reason with less effort.

If in doubt, shut it down

Regardless of the application, when a very loud or ominous sound is heard, shut the machine or engine down immediately. Taking your best guess of the source, perform both a visual and tactile inspection.

Find a pattern

Does the noise appear under a particular load, engine speed, driving state, or when an accessory is engaged? If so, then work through in your mind what is happening during that time.

Is it rhythmic?

This is especially important to consider with an engine or anything else that rotates or reciprocates. Is there a defined cadence to the sound?

Does movement such as a bump in the road or field evoke the sound?

If so, gently apply the brakes during that state to see if it is the brake pads rattling. If the sound goes away, that is the cause.

Keep in mind that noise, especially with metal, travels, and its sound is often changed. It can become amplified, muted, or altered to such an extent that it is deceiving.

Tools of the trade

Your first tools are your hands, ears, and eyes. Feel and try to move suspect areas. Study a potential site to glean if there are any signs of movement such as paint removed, a shiny area from friction, or a bolt that is not fully seated.

When applicable, gently evoke the help of a pry bar to check for movement.

A long extension from a socket set is an excellent way to find a harmonic by pressing it against a suspected area. It will also work as a microphone. In some instances, put your ear to the extension.

An inexpensive mechanic’s stethoscope (around $20.00) is an excellent tool to listen with while eliminating a high percentage of outside noise.

There are also relatively inexpensive (from $125.00 to $200.00) electronic listening devices (wired or wireless).

These are equipped with multiple sensors (channels) and a control console with headphones. It allows for a safe and secure way to compare various sites to look for the noise.

Every machine has its own inherent and natural sound during operation. That is music to the ears of those that operate it.

When something goes wrong, it is no longer a symphony of mechanical apparatus, but instead, just noise!