What happens if you tap the tubes? that is how you check for microphonics. With nothing playing, turn on the amp and let it warm up. tap lightly on each tube with your fingernail. If sound comes out of the speaker, that tube has microphonics. Often tubes with microphonics are seen to be a non-problem and are resold. I don't tolerate microphonics because the problem you are vulerable to is as you describe.
If, OTOH, tapping on the tubes does not cause noise to come out of the speakers, then your problem isn't microphonics in your tubes. In that case I'd say you have a loosely connected ground loop and tapping makes the connection come and go.
Jerry