Is the rattle also coming through the speaker?...if it is that's called 'microphonic' and could also could make a vibrating sound right from the tube, too.
You could rearrange the 12AX7's to get the noisey one in say the driver location. It probably wouldn't be as micrphonically noticeable there. IF the tube is vibrating internally of course it would need to be replaced.
If it is a physical vibration, sometimes you can just pinch the tube with thumb & forefinger & that will silence it indicating that is the culprit. Watch out for heat! Also just tapping the tube with the handle of a screwdriver can bring out the vibrating culprit.
Power tubes can also be microphonic, but ususally with older ones.
We have also noticed that for some louder vibration issues with this style of amp, all y ou have to do is tighten the chassis mount screws that are on the side rear of the amp & lots of vibration will go away.