yeha you had a ground loop...you can change to a separate circuit on the wall or you can lift the ground with on 3 prong to 2 prong adapter. my laptop does the same thing when I plug in my 20 inch monitor to the dvi port while using the core audio, however,once I connect my usb sound card the ground loop is broken. you can also try a di box if nothing else works. since you already fixed it I suppose this is just information for the masses. peaz

and yes to respond to someones question ,separate circuit means another wall panel, for example : the wall plug next to your desk vs the one on the perpendicular wall...many times they are on separate circuits so if you plug the monitors in to one and the computer in to the other there is less chance to create a ground loop that will reach your speakers.