Noise is rated A, B, C, or D in decreasing order of preference. An "A" rated ballast will hum softly; a "D" rated ballast will make a loud buzz. The number of ballasts, their sound rating, and the nature of ambient noise in the room determine whether or not a system will create an audible disturbance.
Ballast factor is calculated by dividing the lumen output of a lamp-ballast combination by the lumen output of the same lamp(s) on a reference ballast. A ballast factor of <1 means that your fluorescent system will produce less light (lumens) than the reference ballast and a factor of >1 means it will produce more light.
Ballasts that can operate more than one type of lamp (e.g., the 40-watt F40 ballast can operate either 40-watt F40T12, 34-watt F40T12, or 40-watt F40T10 lamps) will generally have a different ballast factor for each combination (e.g., 95%, <95%, and >95%, respectively). Ballast factor is not a measure of energy efficiency.
A dimmable ballast is very similar to a rapid start ballast, but usually has a capacitor incorporated to give a power factor nearer to unity than a standard rapid start ballast. A quadrac type light dimmer can be used with a dimming ballast, which maintains the heating current while allowing lamp current to be controlled.
Energy-Efficient Magnetic Ballasts. Energy-efficient magnetic ballasts are simply improved versions of the standard ballasts. Instead of aluminum, these ballasts have copper wire coils, and the iron cores are larger. The copper has lower resistance and the larger iron core generates less heat inside the ballast. These two factors result in efficiency improvements.
Ballasts can be expensive to replace, so first check to make sure the ballast is mounted correctly and that nothing is reverberating off any nearby walls or the ceiling, as that can contribute to the buzzing of a fluorescent light. If the flickering and hum persists, the ballast may be corroded and you may need to replace the ballast entirely.