This might be a long shot, but what guitar are you using and how close are you sitting to your amp when you check? I actually have a similar problem if I’m too close to the amp or in the wrong position with my es-335 because my PAFs aren’t wax potted and I’m in kind of a small room. Do you have another guitar to test out, and does turning guitar volume down fix it? Your hand could be shielding a slightly microphonic pickup. The amp looks awesome, you should be proud! Another long shot: the runs on v2 might be maybe a teeny bit long and could be causing some oscillation with the heaters, so it might help to poke around there a bit. I am probably missing it in your pics, but I don’t see a CT from the PT or an artificial CT with 2x 100R coming off the pilot lamp- not sure if that would cause the noise, but it’s definitely good praxis to include for safety I think. Good luck!