This seems to be a very common problem. Active monitors (and especially Yamaha HS series) have annoying hum/hiss/scratchy hi-freq noise coming that depends on your laptop activity: HDD, fans, CPU load. It’s like listening to your laptop inner noises amplified.
My setup is
Acer laptop + M-Audio Fast Track Pro (USB interface) + Yamaha HS50m
If i unplug laptops adapter from the electricity socket – all noises are gone.
Two solutions that worked for me (in different apartments). Both unexpectedly easy.
1. Plug your laptop AC into a different electricity socket. Once I separated the monitors from the laptop this way – problem was solved. It did not help in the new apartment though.
2. Try a different laptop AC adapter. In my case I used cheap no-name Chinese “replacement adapter” instead of the original Lite-On that came with the laptop. And what do you know, to my big surprise – noises disappeared!
So don’t be discouraged if you have ground loop problem: just try any kind of silly small fixes, it might actually work.
If it still doesn’t go away, check, for example, this:
and google for “ground loop”.