look if they are connected to the same grounds/mains? if the hdd has a power supply of it's own. + move the hdd away from the monitors and see if that betters the situation. make sure the cables going to your monitors are far away from electrical lines (possible the usb cable as well). i had a stupid buzz on my right speaker and even unscrewed a speaker just to find out my telephone (!) was causing the buzz - one of the wireless ones.

if all that fails -do what Mark said :cheese: