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.
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).