Why is a transformer noisy?

Transformer noise is caused by the repeated expansion and contraction of the transformer's steel core. The technical term for this is ‘magnetostriction’. The magnetic fields are generated by alternating current, and take on a wave pattern, following the rhythmic pattern of the current. The expansion of the core is dependent upon the quantity of flux which, in turn, is dependent on the applied voltage and the number of windings on the transformer coils. The magnetic flux passing through the transformer core continually fluctuates, so that the core expands and contracts. This results in the humming sound.