Why Transformer makes "hum" noise

Transformer makes "hum" noise because of Magnetostriction effect. Magnetostriction effect is the property of ferromagnetic magnetic such that change in the dimension of material when the varying magnetic field is applied. 

In electrical transformer alternating voltage is applied to the core. This results in the varying magnetic filed direction in the core. Grain directions will be varied in every half cycle. Due to this varying magnetic field, dimensions of the material will be changed causing the vibrations.

Some of the methods employed to reduce the noise are:

• Preventing vibration of core-plate, which use the lower flux density
• Tightly bolting and clamping the laminations  and structural parts of the transformer
• Sound insulating the transformer from the tank by cushions, padding and oil barriers
• Preventing transformer tank wall vibration by suitable design of tank and stiffeners
• Sound insulating the tank from the ground or surrounding air