Noise and microphonics are different things. Microphonics is what it's name implies. The structure of the tube acts like a microphone and amplifies sounds from outside the tube. If gently tapping the case of a piece of equipment, or the tube itself with your fingernail causes a lot of noise through the speakers, you can say that tube(s) is microphonic. In some severe cases, you'll be able to hear people walking across a floor through the system as the vibrational energy of the footfalls are amplified.
Hissing, spitting, pinging sounds are noise issues unrelated to microphonics. They are simply issues with the tube construction. A tube can be noisy, but not microphonic, and vice versa. Sometimes, a tube damper will control minor tube noise, and some tubes will quiet down after 10-20 run time.
When buying tubes, always request tubes that are low in noise and microphonics. Especially important for preamps that have high gain.
