Check the 100k resistors for sure. Paul’s thought on the speaker is good too, check to see that the speaker frame is torqued evenly to the baffle board, it does not take much to induce “cone cry” when the speaker frame is tweeked. Especially in speakers that don’t have much of an air gap (like most Webers). You said you checked all the solder joints, but a weak joint can do that intermittently for sure.

I had a problem like that with one of my amps once, I replaced the speaker, changed the tubes, removed the chassis, speaker and other cabinet hardware in search of the vibration. After several hours of hunting, I finally found that one of the screws holding the component board to the chassis was causing a sympathetic harmonic at certain frequencies. The screw was tight, but a quarter turn was all it took to resolve the harmonic and the amp has worked fine since then.

BTW, the tea warmer sounds interesting, I’ve not seen one before. I can see why the NOS 5Y3 would be superior to a new manufacture tube. The new manufacture rectifier tubes are just not the same.