Potting transformers for mechanical noise reduction
Grain oriented silicon steel laminations, which are used in building transformer cores, experience magnetostriction when they're subjected to magnetic fields.

That means their dimensions change - when they're subjected to magnetic fields - at a rate corresponding to the frequency of the magnetic field and a magnitude dependent on the intensity of the field.

Affixed rigidly to a structure like a PCB can make the PCB act like a sounding board and transmit the vibrations to the surrounding air and, eventually, to someone's ears.

The cure, ostensibly, is to isolate the transformer from the PCB with resilient "shock mounts" which will damp the oscillations by absorbing them.

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