Been trying to pin down an intermittent hum/buzz on my PR build. It's been totally reliable for a couple of years, but I noticed it was getting noisy. I did the usual easy checks- tightened the socket and pot nuts, new power tubes and solid state rectifier i had kicking around, and thought I'd nailed it. Till a few days ago, when it began to get noisy again. It was only when I went to plug it in to the wall outlet I noticed a rattle from the plug, so I popped it open and found the ground screw was totally loose. A quick tightening up, and now all is good. It's my indoor amp, and hardly ever gets unplugged which is why I'd never noticed it before, and still can't understand how it worked loose, but at least I fixed it before getting a shock!