You find this a lot, especially with combo amps. Mostly, because the quality of new tubes being made today is poor. No way around it, they just don't make them like they did 50 years ago. The new ones rattle more. A well known, and respected amp maker, who's name may be the same a city in Arizona, readily admits this. There are tube "sleeves" or dampers, or something that you can get to correct this. If Les chimes in, he can tell you for sure. He's put some on one of his amps for this very problem.