Resonance

Resonance:
Resonance is a sound that is created by vibrations. A vibrating object will cause resonance in another object. One example is resonance in the engine compartment. Due to the vibrations of the engine, for example a bolted connection or a small bracket of the air filter housing can start vibrating as well. This creates a resonating sound that can be heard at certain engine speeds. The vibration frequency is in fact dependent on the engine speed.

To find out which object is causing a resonance, one person can hold the engine at a certain speed and listen, while another person grabs components in the engine compartment. By holding the air filter housing (as in the example above), pressure is exerted on the bolted connection and/or its bracket. This damps the vibration and the sound disappears.
After that, a solution can be found to counteract the vibration, for example by fitting extra washers, bending a bracket, or applying insulation material.