Resonance

Resonance:
Resonance is a sound that arises from vibrations. A vibrating object will cause resonance in another object. An example is a resonance in the engine compartment. Due to engine vibrations, for instance, a screw connection or a bracket of the air filter housing may start to vibrate. This results in a resonating sound that is audible at certain engine speeds. The vibration frequency depends on engine RPM.

To find out which object causes resonance, one person can hold the engine at a specific RPM and listen, while another person holds parts in the engine compartment. By grabbing the air filter housing (as in the example above), pressure is applied to the screw connection and/or the bracket. This dampens the vibration, causing the sound to disappear.
Then, a solution can be found to counteract the vibration, such as by installing additional washers, bending a bracket, or applying insulation material.