I'd start by cleaning both ends of the FO cable if you haven't already. It could be just a poor connection that is being rattled a bit by the sound. If you have the option of using a dig coax you may want to try that. If not, & cleaning + re-terminating the original FO cable doesn't change anything, then try getting another FO cable. A higher quality cable may fit better, insuring better transmission. If that does not work, try using another port on your receiver.In fact,try that before you go spend money on another cable. If you have another FO port that nots being used,even if it's assigned for something else like a CD or DVD player,you can try it. If all fails I'd suspect the HD box.
Does it only happen when watching TV? Or does it occur when listening to music as well?
