Are the "big resistors" microphonic? Over the years I've seem almost any component - 'cept a transistor - become microphonic. Capacitors are the worst of the passive components but resistors - particularly carbon comp and wirewounds - have become microphonic. Of course tubes are the worst and I truly remember a 6SL7 that I could actually sing into (the fellow who owned the amp stated that it was an "improvement" in my singing voice) but I would suspect some other component that vibrates when you tap the tubes. Hell, change the resistors anyway - they're cheaper than the tubes and perhaps there are poltergeists hiding within.