Microphonics of tubes

Since September I have my first tube component ever, and I have noticed interesting phenomenon (besided much better sound then from my SS predecoessor):
When I tapping chassis when tubes are hot (at least 1h amp have to be turn on) loud sound about 1-2kHz can be heard. When they are cold there is nothing. It is happening only with one of the tubes (I have moved microphonic tube between canals), so this is definitely future of tubes not preamp?
Even preamp (Supratek Chenin) is using 4 different tubes only 6SN7 is microphonic, bigger 5881 are not.
I have tried it with just bought Ken Rod VT231 (replacement for 6SN7), the some story one of the twos is microphonic, but sound is lower in frequency. I have question is it mechanical problem, poor construction of the tube (something loose inside) or it is electric problem (they are microphonic only when they are hot).
What is exactly nature of the problem. Does it mean if they are microphonic that something is wrong with them ? Or maybe it does something to do with pre construction (meaning in different pre they would not be microphonic), because other tubes are not microphonic at all ?
Sorlowski, what you are experiencing is simply part of life with tubes. As you've discovered, the microphonics is due to the tube, not the equipment. And, it can be variable from tube to tube. The cause is the internal construction of the tube: the parts inside the tube can vibrate and when they vibrate, noise will pass into the audio circuit of your equipment (depending on where the tube is in the circuit). The noise is the result of mechanical vibration, it's not electrical. And it's not necessarily a construction quality issue. But it can certainly be variable from tube to tube coming off the same production line depending on the tolerances of the contruction process (which some would argue IS a quality issue, but of a different sort).

It is possible to test for tubes that will be less microphonic, but it requires specialized equipment.

Generally, the answer is DON'T TAP ON THE TUBE! ;-)

The fact that a given tube is microphonic does not, of itself, mean that there is something wrong with the tube. If you don't hear noise during normal operation of your equipment, it's not a problem.

Also, there are all sorts of after-market tweaks for trying to reduce the internal mechanical noise of tubes. That becomes a whole additional universe of things to try to see if any of it makes an audible difference in your particular system. Some will.

Hope this summary helps a bit.
Yes, it helps, it calms down me :)