I would never suggest hitting the back of an electronic device as an efficient way to resolve a problem... maybe you should kick it?
Just kidding. Sounds to me like a connection problem. As Jefferson mentioned, car or home stereo would help a bit. Either way, usually if there is a loose connection with any of the speaker wires, you will experience "pops." The fact that it stops when you hit it, shows that your interaction is changing the overall connection of your stereo. When it does stop and you go back to watching, it is probably the speaker vibrations that are loosing the connection again. If nothing changed when you hit it, I would expect it to be an electrical issue. My suggestion... pull it out, re-wire it from scratch... making sure that all metal is connected and not in a position that it can contact any other metal. It might sound like the last thing you want to do, but putting out the flames of your freshly scorched receiver is much less fun. Hope that helped a little. There has to be an audio specialist on this forum somewheres... Stay and Keep :jointsmile: