Why do older fluorescent lamps sometimes make a humming or buzzing sound?

The humming or buzzing sound associated with older fluorescent lamps is caused by the ballast, a device necessary to regulate the voltage required to ionize the gas within the tube.1

In these older systems, the ballast is a separate component located within the lamp fixture. As it operates, it produces the characteristic audible noise. In contrast, modern compact fluorescent lamps utilize electronic ballasts integrated into the base of the bulb, which significantly reduce or eliminate this buzzing sound.1