Why can a transformer hum, and is its main hum always at the supply frequency?

Solution & explanation

Magnetostriction and magnetic forces make the core and windings vibrate. Core dimensional change commonly has a strong component at twice the mains frequency, so 50 Hz supply can produce 100 Hz hum, with other components possible. The box’s simple 50/60 Hz statement is incomplete.

ABB research · transformer core noise ↗ ·