Pinging sound from Preamp

I just re-arranged my system, and you got it, the audio gremlins have struck. Immediately after start-up mute cycles off on my BAT VK3i, I hear a rhythmic "ping" from the pre and through the speakers. After starting CD, I cannot hear it, then after a while (30-45 min) I start hearing it through speakers over (lounder than) the music (which is at a fairly low volume). When I mute the pre it goes away, and it is present regardless of source selected (even if source is shut off). Any thoughts?
The audio gods have rained on you :>(

I am interested to know what you find.

Once, on an old pre, a similar problem was a flake of metal in the selector switch. Good luck.

Had a similar problem with an amp. Turned out to be a faulty capacitor in the mute/protection circuit. I don't know enough about the BAT to comment on what may be the cause of your problem, but thought I would share my own experience.

Regards, Richard.

After re-reading my post, maybe I should say pinging starts AFTER pre-amp goes thru start up and is no longer muted, i.e, when pre-amp is muted, no pinging, when pre-amp is active, pinging is heard. I've swapped out the 6922s to a new set of NOS Amperex and its still there; now on to the 6V6s (which don't have many hours on them).
Mike that sure sounds like a microphonic tube; seems like you're on the correct path toward isolating it. Very strange that it suddenly appeared in the manner that it did, & that changing tubes didn't help; really makes you wonder?