Turn each circuit on and off until the sound disappears. There is your problem circuit.
If that does not work then you may have to change your main breaker to get it to stop. This sound is not necessarily normal but does happen from time to time.
If that does not work then you may have a problem with harmonics and may have to re-evaluate the size of your neutral on your panel.
If your panel is a Federal Pacific Stab Lok then it should have been changed years ago, nevermind what I posted above if it is.