It seems to stop when I reconnect the pot to the case. I had it out so that I could test it. I ahve since bolted it back in to the case. It's very weird. At one point, I was measuring 0.6VAC between the case and ground, which made no sense, unless RF can really cause .6V, that was with the case ungrounded. It's really inconsistent, sometimes it happens when I touch the case, other times, when I'm not. It happens with the case grounded, then I'll disconnect the grounding wire and it will stop. Then, any time I think I've got it figured out, it will do it with the opposite, so when it will start with the case ungrounded, too. Touching the tubes sometimes makes it stop. I cleaned the contacts, thinking that they might be dirty, and that seems to have helped marginally. Every resistance and connection inside is as it should be. I resoldered all of the tube socket pins, just in case, and that seemed to help marginally. I'm really at a bit of a loss. If connecting the volume pot to the case really gets rid of it, then I have 0 ideas what caused it.
With the volume all the way down, there is also a buzzing. I ahve tried switching tubes, and it always stays in the right channel.