Could be that the tweeter is damaged, or something electronic is being reproduced by the tweeter.
Well it took a day or so but I figured it out. It was something electronic vibrating in the amp afterall. When it was making the rattle sound, I got it to stop by picking up the jack-side corner of the amp. Then I checked the tubes and the socket under V2 is a little loose. The socket itself seems to have some play, not the tube within it. Is this broken or something I can fix?

