When I run run my laptop off AC and dock it so it's connected to my powered monitors (M-Audio StudioPhile SP-58s) via my USB sound card, I hear a buzz through sparkers when the heads on my hard disc seek. I hear this with both an M-Audio Fast Track Pro and my Novation NIO. By lifting the ground on the speakers using a two prong plug adapters I isolated the issue and determined it was in fact a ground loop issue.
The Hum X Exterminator on Musicians Friend claims to be a safe alternative lifting a ground with a two prong adapter.
According to the product description the Hum X "...removes unwanted voltage and current in the ground line that cause ground loop hum. This noise reduction adapter simultaneously maintains a solid, safe ground. You no longer have to run your audio signal through filtering that results in loss of volume, tone, or both. Some devices simply remove or float the ground, which is never safe! Hum X removes the ground loop while leaving the ground and your signal intact. Easy to use and completely effective!"
I've not used the product but think it sounds like an interesting idea. If you have one, leave on comment on this post and let other Modulate This readers know your opinion.