Why the transformer make noise during operation?

➽ The basic cause of transformer noise is magnetostriction.

➽ There is expansion and contraction of the iron core due to the magnetic effect of alternation current flowing through the transformer coils.

➽ The magnetic sheet steel extended when magnetized again it goes back to its original condition as the magnetization is taken out.

➽This produces an audible hum. 

➽Magnetostriction may be partially controlled by the transformer design, but it cannot be totally eliminated.