All modern ESCs work by switching the current on and of very rapidly to control the amount of energy put into the motor over any given period of time. The early ones were tied to the frame rate of the transmitter, about 50Hz, and growled a bit at low speed. The present day ones are computer controlled, and run at a higher frequency, so at low speed the motor can respond to this. Some motors are more susceptible than others. Most of us damp the sound by using a resilient mount, rather that doing our best to transmit the sound into the hull structure, which amplifies it. Some more modern ones can vary their frequency, with advances in technology, some can run at a high enough frequency that humans can't hear it.
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