"Magnetic ballasts modulate electrical current at a relatively low
cycle rate, which can cause a noticeable flicker. Magnetic ballasts may
also vibrate at a low frequency. This is the source of the audible
humming sound people associate with fluorescent lamps.

Modern ballast designs use advanced electronics to more precisely
regulate the current flowing through the electrical circuit. Since they
use a higher cycle rate, you don't generally notice a flicker or
humming noise coming from an electronic ballast. Different lamps
require specialized ballasts designed to maintain the specific voltage
and current levels needed for varying tube designs."
more about flourescent lights at:
http://home.howstuffworks.com/fluore....htm/printable