All fluorescent lights require a ballast to function. All ballasts hum to some degree; both magnetic and electronic fluorescent ballasts will give off a slight humming noise. Magnetic ballasts tend to hum more than electronic.

By design an electronic ballast typically makes less noise than a magnetic ballast. You may hear them in a quiet room, however, in most applications, noise from an electronic ballast is covered up by the usual ambient background noise in a typical room.

A loose magnetic ballast can be the cause of ballast hum. Check that ballast mounting screws are tight. If the hum is louder than usual the ballast may need to be replaced as ballasts can wear out over time. If you do choose to replace a magnetic ballast, consider replacing with an electronic one.