[Reece]

Coot has it. Tap each tube in turn and you should localize it. The answer is a new tube, but you can often silence a microphonic tube with extra weight attached to it, which modifies its mechanical point of resonance. A way old timers did it (thank you, thank you) was to wind a number of turns of heavy solder around the tube envelope. Then slide it off, tighten up the turns a bit, and slip back on so it stays by its own pressure. Looks funky but usually works.