Tubes becoming microphonic

If you gently tap a tube with a rubber pencil eraser, and you can hear the sound of your tapping amplified through your loudspeakers, then your tube is microphonic.

I once owned tubes that were so microphonic, they would amplify the sound of my fingertips brushing across the volume knob of my preamp.

Almost any signal-level tube in a line-stage exhibits some microphonic behavior. It's a matter of degree. If the tube is really bad, there will be a shimmer or whistle in the sound or bass thickness introduced by chassis or air resonances. If wrapping the chassis doesn't make loud noises through the speakers I wouldn't worry too much.

Microphonics is one contributor to "tube sound". In moderation it is not unpleasant.