Resonance is a sound created by vibrations. A vibrating object will cause a resonance with another object. An example is a resonance in the engine compartment. Due to the vibrations of the engine, a screw connection or a support of the air filter housing, for example, can also start to vibrate. This creates a resonating noise that can be heard at certain engine speeds. The vibration frequency depends on the engine speed.
To find out which object is causing a resonance, 1 person can hold the engine at a certain speed and listen, while another person grabs parts in the engine compartment. By gripping the air filter housing (like the example above) pressure is exerted on the screw connection and/or its support. This dampens the vibration and the sound disappears.
A solution can then be found to counteract the vibration, for example by mounting extra plate rings, bending a support, or by applying insulating material.