Magnetostriction

Magnetostriction is a property of ferromagnetic materials that causes changes in physical dimensions and shape during magnetization of these materials. As the applied magnetic field changes, the dimensions of these material also
changes slightly. The humming noise in a transformer or a ballast of a fluorescent lamp is due to the property of magnetostriction.

In transformers as alternating current is applied, it establishes alternating magnetic field in the core. This alternating magnetic field causes continues changes in the dimensions of the laminations in the core of the transformer. The core material expand and contract in each cycle of the applied magnetic field. These changes in physical dimensions cause friction and produce noise in the transformers. This noise is called transformer hum. For transformers working a power frequency as in switch mode power supplies they produce objectionably high noise.