Transformers usually make a humming sound. Everytime the magnetic field changes, it can cause the transformer to vibrate. 60Hz is a low frequency hum, but higher frequency transformers give off a high pitch sound. At my job, only the younger guys can hear the whine of the higher frequency, it’s beyond the range of older people ears.
But other parts can also make sounds. A capacitor makes a sound when quickly charged or discharged as well.
Large transients, like turning on a high powered device, can cause large inrush currents into the tx, making it hum more loudly until the load reaches a more steady state.
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