Answer / koti1965

Due to Magnetostriction.

Answer / aarya

Most transformers make some audible sound at the frequency of the current they're operating on, for transformers running on utility power in the US this is 60 Hz. The sound is due mostly to the wire in the transformer coils and the steel in the transformer core vibrating at 60 Hz because of the interaction of the magnetic fields of these components. Some of this sound may also be due to something called magnetostriction in which the steel of the core actually expands and contracts with the fluctuating magnetic field.

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