How do resonance noises occur?

Every motor, whether for washing machine or electric grill, generates cyclic vibrations.
These vibrations are acoustically imperceptible by human hearing.
However, they are transmitted via solid objects that are also set into vibration. Soft, damping materials can interrupt the course of the vibrations.

If such a vibration hits a resonance body, e.g. a wooden or plasterboard ceiling that is not stably suspended or a loose beam, then this resonance body produces a possibly very unpleasant noise, the so-called resonance hum.
It is the principle of the guitar: The string itself vibrates inaudibly. Only the wooden body (sound box) makes the sound audible by resonating.

As a countermeasure, the insertion of foam rubber strips in the suspension (hemisphere joint) or between the ceiling and the attachment is a good idea. There must be no more rigid connection point to the ceiling! Furthermore, when selecting the mounting location, make sure that the attachment is screwed to a cross beam, a batten or a support element, if possible.

To test the cause of noise, it is advisable to hold the motor (with connected grounding and in compliance with all safety regulations) freely in the hand without blades and let it run in all stages. The motor must not rest on anything during this process. If the noise can still be heard, then the motor is defective. If it has disappeared, then the noise was caused by resonance, which is to be combated with the above measures.