A transformer helps move electricity from one location to another. During its working, a transformer may vibrate or shake a little, and that tiny movement can cause a humming noise.
The buzzing sound commonly heard from transformers or fluorescent light ballasts is caused by a phenomenon known as magnetostriction.
This effect is a result of the mechanical response of the iron core material to the magnetic field it experiences.
In the case of U.S. circuits where the standard frequency is 60 Hz, the hum produced will be at 120 Hz due to the ferromagnetic properties of the material. The maximum length change occurs twice per cycle for 60 Hz AC devices, such as transformers, resulting in the familiar and sometimes 120 Hz hum.
The Basic Theory Behind Magnetostriction
Magnetostriction is a phenomenon in which magnetic sheet steel extends and contracts when it is magnetized and demagnetized.
In the case of a transformer, the magnetic core is excited using alternating voltage and current thereby causing it to extend and contract twice during each magnetization cycle. These extensions and contractions are not uniform and vary across each sheet of the core.
Transformer cores are constructed with many thin sheets of special steel to minimize these effects and reduce energy losses and heating.
Since a transformer core comprises of many sheets, these erratic extensions, and contractions can cause the entire structure to vibrate, resulting in noise. Although these extensions are too small to be seen with the naked eye, they are sufficient to create a vibration that generates audible noise.