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How to find an exhaust rattle using a rubber mallet

December 30, 2018

So you suspect a rattle in your exhaust system, but how do you know which heat shield or which component is the culprit?

95% of the time a true rattle that exhibits itself at certain RPM’s is attributed to a loose exhaust heat shield. The sound is most pronounced at specific RPM’s because of resonant frequency. In more laymens terms; when the vibration frequency most closely matches between the engine and the exhaust metal is when the most amount of vibration is experienced.

Vibration = sound in this case.

After lifting the car, the best way to reproduce the sound and then subsequently find the sound, is by tapping the exhaust pipe using a rubber mallet.

Here is why it works so well:

Steel has a high Young’s modulus GPa which is a measure of elastic stiffness.

What that means is that when hit, it propagates energy without significant loss, and much like a newtons cradle, when hit on one end will transfer most of its energy to the other end. On the other end in our case is a loose exhaust shield.

Rubber on the other hand has a very low Young’s modulus GPa, which means that although it is visually elastic, it is not very stiff. These sort of materials lose energy through absorption (heat) and therefore do not make a sound and do not propagate energy very well.

When searching for a steel on steel rattle, it makes sense to impart energy by rubber, which on impact will not make its own sound, but will exert enough force to allow the energy to propagate through the steel pipe and then to the loose heat shield. Further, the rattle can be heard for a longer duration because rubber will exert force over the period that it deforms.

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