Why not turn your computer off or unplug it.This problem is most noticeable in the high frequencies.Don't forget about the flourescent lights!
Can Computers add noise to home electrical system
I just read something that some audiophiles feel home computers can add noise to your homes entire electrical system, despite having dedicated lines for Audio, and their may be some benefit from trying to isolate noise where your home computer is set up, with something along the lines of maybe quite lines, a noise hound , or an ultimate outlet. I am going to try this , but wonder if anyone ever experimented with this . Regards.