Humming sound of transformers

      The humming of transformer is due to the phenomenon known as magnetostriction.This is the phenomenon which cause a piece of magnetic sheet to extend itself when magnetized.When the magnetization is taken away it goes back to its original shape.
   In a transformer the magnetic field is produced by the applied alternating voltage,hence the magnetic field set up by this alternating voltage also alternating in nature.So for a full cycle of this magnetization, the sheet laminations of core will expand and contract.The magnetization at different points on the sheets are different hence the expansion and contraction is not uniform.Transformer core is made up of large number of sheet laminations to reduce eddy current losses.Each laminations have this expansion and contraction.We cant see the variations with our naked eyes, but they are enough to make vibrations and produce noise(hum).Applied voltage produces the magnetic flux,this flux produces humming sound in transformer.Hence the level of humming is depend on the applied voltage.