Sounds like a ground loop, search on the htpc forum or the entire avs forums about "ground loop".
|Originally Posted by dhiren|
Im not to sure if this helps you but i kept getting a buzzing sound from my tv (sharp lcd da5u) specifically when connected to the pc via dvi >hdmi
What solved the problem for me was to tell the tv to assume that hdmi connection was bringing video as well as audio which stopped the buzzing,
The TV was initially set to process only video from the hdmi connection