I often get inquiries that start out something like, "my tubes must be microphonic, because whenever I tap on them I can hear it through the speakers."
Amplified tapping does not indicate "microphonic" tubes. Some tubes will be more sensitive to being tapped on, others less so. Small-signal gain tubes will typically respond to tapping, power tubes not so much. Sometimes a tube that is so microphonic that it isn't even listenable may not respond to tapping, while a very low-microphonic tube will amplify tapping horrifically.
Tapping on tubes can damage them, loosen the internal parts, and make them go "microphonic."
The only thing that matters, really, is how do the tubes perform while listening to music. A tube doesn't necessarily have to scream out at you, saying, "Hey, I'm microphonic!" by whistling, reverberating or making lots of weird noise. Virtually all vacuum tubes are microphonic to some detrimental degree. Unwanted microphonics add fuzziness to musical detail, blur the focus, add glare, and cause other distortions. In addition to good component isolation, damping instruments reduce these distortions, allowing the more natural and real music to come through. In many cases, the less microphonic a tube is, the more it appreciates a damping instrument because it then performs even more superbly.
Poorly designed tube dampers or simple O-rings often damp not only some micro-vibrations that cause distortion, but also tend to damp the music itself by introducing resonance and other anomalies to the microphonics. On the other hand, properly designed damping instruments damp unwanted vibrations only, allowing an unfettered frequency response and effectively undamping
the music by allowing its true nature to be more fully revealed.
Unless you have a leprechaun hiding behind your stereo rack who taps on tubes while you're listening to music, there shouldn't be any particular concern if your tubes react to tapping on them. Nor any reason to tap on them in the first place.
SteveHerbie's Audio Lab