I have also had this problem several times in the past. Power conditioners, filters, cords, and such will have absolutel no effect on this problem. For a cheap fix, you can go to RadioShack and get a "ground loop isolator". This is basically as set of cheap RCA's witht he isolator on them. It works 100%. If you want t spend the extra money you ca get a MAGIC. For details on the MAGIC, as well as info on eliminating these, you can go to Michael Elliot's website, altavistaaudio.com, and go to the hums and buzzes section. He has links to retailers of the high end tiems. After years of struggling with this problem via expensive audiophile/videaphile fixes, and even disco'ing the cable when listening to music, a friend brought over the $8 Radio Shac fix and it worked like a charm. Hope this helps.
Cable TV causing system ground problems!
After responding to another thread about powerline products. ..I wanted to post what you might call a warning...or at least a "heads-up". Cable feeds...be they for TV or cable internet..or FM, can cause real problems for your stereo. The ground on cable can have DC and other problems..that if connected to your system through interconnects from unit to unit, or to the AC can cause you to go searching for a cure. The real cure is to keep cable feeds out of your system and your systems AC.