They are a brushless DC motor. There is a driver or electronic module that conditions the DC voltage to have alternating pulses to run the motor. The module's output frequency for the motor is regulated by the radio control system.
You are hearing a combination of the electric pulses and the pulses of the motor as it runs. Also the propeller blades make a noise as the tips approach or exceed the speed of sound.
Makes it sound like a little two stroke nitro engine running 25,000 rpm?
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