All ESCs work by switching on and off rapidly. Older ones do it 50 times per second, and give DaveM's low growl. More modern ones run at a higher frequency, and cause most motors, beacuse they have most of the components that loudspeakers have, to act as loudspeakers and make a sound at that frequency. Vipers, and many others, are bang in the middle of the human hearing frequencu range, so we hear a whistle. Some ESCs work on yet higherfrequencies. Humans don't hear them, but they might attract the attention of dogs being walked in the park.
Personally, I don't mind the whistle, it lets me know that stuff is working.
Brushed ESCs generally work on one set frequency, and to make the motor go faster, make the pulses longer, offering more power.
Brushless ones need to work at a changing frequency to pulse the motor coils at the right speed. They probably produce plenty of sound, but with everything else going on as speed builds, nobody notices.