One of my co-workers had a resonant frequency on an immaculate '66 Fender Pro that he had just bought (uh-oh). The only way we could diagnose was for me to "crank" the notes and for him to carefully check the cab/chassis by hand pressure. It was the electrolytic, filter caps which are on their own little board and covered with a metal cover on the bottom of the main chassis. Fender must have known that the small board with the caps would vibrate so they put a strip of padding between the caps and the metal cover. Over time the padding had dried up and allowed the small board and cap assembly to vibrate. We literally shimmed the padding on the inside of the cover back up to hold the caps in place and the resonant frequency was gone. Can you get someone to play those F#s and Gs while you check?