If your flyback transformer has a pinhole in it, there will be snaps and flashes of light, but only when the set is on. If one of the parts in the primary circuits of the main power supply is arcing, it will stop once the plug is pulled from the wall. If you dim the lights in the room and take the back off, or look through the ventilation slots in the back, do you see any flashes of light, sparks or any type of arcing? You said the noise continues when the set if off and unplugged. Do you mean it keeps on going and going, or does it only continue for a second or two?
Usually answered in minutes!