| You're saying something inside the head itself is rattling, right?
As a brief test, take the amp off the cab and put it on a padded chair or something -- anything that will acoustically decouple the head from the cab. Make sure not to block any air vents. If the buzz goes away, then yes, something inside has rattled loose. The next step would be for someone to go inside the amp, but with a tube amp this can be dangerous even with the amp unplugged (if it was recently powered up) if you don't know what you're doing, so I can't really recommend you yourself do it. Might be worth an hour of tech time to have someone go in for you and sock everything down tight, and put Loctite or lockwashers in as appropriate.
After the problem's fixed, run your amp with some sort of vibration isolation, again being careful not to block any airflow. | 
