You can pickup a line noise filter at advanced auto parts. They are made for car stereos, It hooks up to the +12v and filters out noise caused by the alternator on the car. They can carry around 20 amps and they only cost $10. Just connect it to the constant +12V connection on your PSU. In the past this has been the solution to line noise problems I couldn't fix by any other methods. If all else fails try getting a new soundcard. That has also worked in the past. Just my $0.02.
Until next time, drink and be merry.