This is from
http://www.passandseymour.com/knowhowfaq/showquestions.cfm?faqcategory=Dimmers%20and%20Controls , Pass & Seymour: Knowledge & Know How: FAQ
Q: I installed a dimmer at a new house and the owner is complaining that the incandescent light bulbs are humming. I changed the dimmer and the bulbs are still humming. Is there a problem with the dimmer or the installation?
A: Why does a light bulb hum with a dimmer controlling it, is a very common question. The noise the bulb is making is called "bulb sing" or "filament hum". The sound is created from the turning on and off of the A.C. sine wave. The rapid switching causes the tungsten filament to resonate. This is not harmful to the lamp but could be annoying depending on the location of the installation. Filament hum is typical with what is referred to as "contractor bulbs". This is an inexpensive lamp that is common to new construction. The hum can be greatly reduced by upgrading the lamp. By using an incandescent lamp with a heavier filament, the hum is practically eliminated. The family dog can still hear it, but he doesn't care.
