Why do my new dimmable CFL bulbs hum?
It looks like CFL bulbs are difficult to dim.


  A dimmer typically reduces the effective voltage, which decreases
  current to the bulb. The science behind fluorescents just isn't meant
  to work that way, explained Paul McLellan with eLightBulbs, a division
  of Service Lighting, a Maple Grove, Minn., lighting supply company.
  
  It can cause dimmer switches, or the ballast, to buzz. It can even
  affect other electronic devices nearby, such as radios. The distortion
  can even go so far as to cause flickering in the light bulb itself.
  Occasionally, you may be able to dim by 30 to 40 percent without any
  adverse reactions.
  
  "We have looked at around 20 different brands and although some of
  them get close, the science is not yet there," McLellan said. "We do
  hear of some success, however, usually with high-quality, brand-name
  bulbs and dimmer switches." Sometimes it's a matter of finding which
  brand of bulbs works with specific dimmer switches. You may need to
  change the dimmer.
  
  Remember, not all compact fluorescents are designed to work with
  dimmers, but CFLs made for dimmers are available. You can use regular
  compact fluorescent bulbs with dimmer switches, but doing so can
  shorten bulb life and nullify the bulb's warranty."


Taken from CFL Dimming Article