Have you tried plugging in the headphones by themselves as Marc said though DS?
I don't know alot about grounding and power setups, but I do know over here in Japan the standard wall power out-lets do not have earth. To stop the little bit of noise I had in my setup, I made my own earthing sytem running the mixer and speakers on the same earth cable (infact most things go through the same earth cable). This stopped the small amount of noise I had straight away.
I'd try the headphones DS, if there is no noise, then read up about earthing/grounding your gear.
