I had that happen back when I used a CRT and it turned out it was because my guitar stack was on the other side of the wall in the other room. Those 12" speaker magnets are killers.
Oh man you fixed it for me! I just got some Logitech speakers and I put the subwoofer right behind the TV, almost on top of it. I took it off and now its fixed. Thank you so much! The word speaker tipped me off.