Two sounds have frequencies that are too close together. There is interference and they create a third sound that isn’t too pleasant.
Beat Frequency, harmonics, or resonant frequency.
Beat frequency refers to the interference pattern created when two sound waves with frequencies close to each other interact. This can result in a beating or pulsating sound that is generally considered unpleasant.
Harmonics refer to the multiples of the fundamental frequency of a sound wave. When two frequencies are close together, their harmonics may also interact and create a dissonant sound.
Resonant frequency is the natural frequency at which an object vibrates most easily. When two frequencies are close together, they can interact with the resonant frequency of an object and create unwanted vibrations and sound.