If you unhook the cable box and the problem disappears, then the problem is coming from the cable TV. You can further verify it by only disconnecting the RF cable from the cable box (leaving it completely hooked up otherwise). If the problem is related to the grounding on your cable TV service, there are several products that you can buy to put in line with the cable TV signal to alleviate the problem. I did a search through some old threads in the forum and came up with a few ranging from the expensive side to the very reasonable: MIT ISO-LinQ (probably on the pricy side), Mondial Magic Box (also pricy at $100), Dayton isolator (at $10 or so from Parts Express, this is an excellent thing to start out with if you do determine that you have a ground problem), and the Jensen VRD-1FF Cable TV RF Isolator (available from Markertek for $50).