Tubes becoming microphonic

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.

"microphonic" implies to bad or poor tube that needs to be replaced. you can also tap on amp or preamp chassis to get the same microphonics in that case so you should carefully examine which tube is realy give troubles 'cause you may tap on one tube and another tube will "respond"...

My experience with microphonic tubes was I first heard my volume knob being turned in my speakers, with a slight "ping" sound.

Thanks for your responses, I now have an idea of what it means, and how to test for this condition