Correct Answer : Option (C) : Magnetostriction in an iron core
Explanation : The noise of the transformer, also called humming, is mainly due to two reasons
Magnetostriction : Magnetostriction refers to the expansion and contraction of the laminations (iron core) due to the magnetic effect of the current flowing the coil of the transformer. Magnetostriction may be controlled, but it can't be totally removed depending on the transformer's design.
Stray magnetic fields : Generally, the steel used in the transformer core and wire used in the transformer coil vibrating at 50 Hz to 60 Hz frequency because of the interaction of the magnetic fields.
