Persistent ringing is more common in small preamp & amp input tubes than in large power tubes. Flicking your fingernail (even somewhat sharply) against the tube, will usually reveal the culprit and eliminate the ringing for awhile. The juggling vibration of removing and rotating the tubes will probably also eliminate the ringing for a time. This kind of ringing is different from microphonics, and probably cannot be eliminated with damping rings.
Dave
Dave