Tubes becoming microphonic

"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"...

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

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