It would also help to take the amp with you to a different location, as sometimes noises like these are a consequence of some other equipment polluting the environment around you, and the amp itself is just picking up stray noise that may not exist somewhere else. If that ends up being the issue, then there is a whole different set of tests to do to come up with potential solutions to reduce that to a tolerable level.
Oxidized tube pins can also cause problems like this, and it can help to take a scotch-brite pad and very gently rub it across the pins to be sure they are relatively clean.