Tubes becoming microphonic

I own a CARY SLP-05, this is my first experience with tube equipment. I recently read a thread and the person mentioned his tubes became "microphonic". How can you tell when your tubes become "microphonic". What are some of the leading signs. Thanks
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.
"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