I have a small shop hand wired 20 or so watt amp. When I lay of a note during the decay of the struck string/note the amp makes a popcorn sound. Granted the amp is pretty darn loud for this to take place. I am wondering if it is a loose connection someplace. a tube (relatively new JJ's) both power and preamp. Anything else? Could this be normal? If I turn down the volume a bit it seems to go away.