The glowing filament inside an incandescent bulb is an electromagnet. When the AC current flows the filament develops an oscillating magnetic field, 60 magnetic cycles per second.

With full brightness the AC current (not voltage, current) through the filament ramps smoothly in a sinusoidal manner. Sometimes a bulb filament will vibrate sympathetically with this alternating magnetic field even at full brightness but generally the sound is inaudible.

If you bring a strong magnet near the bulb surface you may be able to see the filament vibrate visibly.

When the power to the bulb is 'electronically' dimmed the previously sinusoidal AC current waveform becomes bastardized. The magnetic field now jolts and collapses repeatedly, 120 times per second. The glowing filaments can vibrate sympathetically and conduct the vibrations to the lamp base and the fixture.

Do the lamps that buzz when dimmed have a filament design that zigzags or loops around inside the bulb? Sometimes that can magnify the magnetic resonance effect and become more audible.

The bulb construction is such that it acoustically conducts the filament resonance more than a different brand or bulb filament design. If you find a different bulb that dims with less sound, change the bulbs.

Curiously, here is a patent describing the magnetically indused vibration of the support wires for the filament. I found this interesting; The frequency of the noise emitted by the lamps was in the range of 4,000 to 15,000 Hz.

That would be very much in the range of human hearing, but at the high end.

Tips from around the Internet;
Lutron Lamp Debuzzing Coil (LDC)

http://forums.mikeholt.com/showthread.ph...8921#post958921