I would bet the majority of people with ht systems deal with ground loops, interference or other related problems at one point or another. Ground loops seem very common to me (that is atleast my impression). I had to deal with it after adding a second sub. The hum was coming through the subwoofer and not the speakers.
Also at one point I noticed a moving horizontal line on the LCD only when using the xbox 360. It turned out the computer in the next room caused this when it was on while playing the 360.I moved the computer upstairs and the problem was solved.
As of this moment my system has no "noise" and I hope it stays that way but you never know whats going to happen when adding new equipment in the future. These types of problems can be a real PITA.
