Seems like a ground loop to me. I also had this issue with my laptop - horrible buzz.
What you can do:
- Try not to get the power all from one source. If it's possible, try to use different power circuits.
- This didn't help for me since the room I'm in has only one power circuit so I bought this:
http://www.behringer.com/EN/Products/HD400.aspx
All buzzing gone.
Note:
I only need this for a Live setup and for this the sound is fine for me with the HD400. Haven't made extensive tests if there's small sound degredation with this device that could make it useless for studio work. But must online dealers offer a return policy, so you could test it before.
Note 2:
There are also tips floating around the net to cut the ground pin on your computer or use a three to two (ungrounded) power adapter.
I'm not a expert in those things but I've read several times that this is dangerous. So don't do this.