Zero crossing dimmer is key. Cheap dimmers cause all kinds of noise. Here is what I learned posted in a non audiophile forum...
http://www.gearslutz.com/board/geekslutz-forum/838320-buzzing-sound-ferrite.html
J
http://www.gearslutz.com/board/geekslutz-forum/838320-buzzing-sound-ferrite.html
J