What is the source of sound during the flight of a bee, wasp, beetle?

Sound is called the vibrations of the air perceived by our organ of hearing, caused, in turn, by the vibrations of some mechanical body. In this case, the frequency of body vibrations should be in the range from 20 vibrations per second to 20 thousand.
When bees, wasps, beetles, mosquitoes and other insects are flying, their fluttering wings are the source of sound. Moreover, the higher the vibration speed, the higher the sound. And, conversely, at a low vibration frequency (more rare flapping of the wings), the sound is lower, as in beetles, for example.