I have a large number of 12au7s, and it does this with all of them unless it is cold.
I basically only listen to vinyl, so this evening I switched from the vinyl input to the next one on my preamp. It still beeped. I turned the preamp off, and it still beeped. I pulled all the 12au7 tubes on the st70, and it went away! I replaced the outer two and it did not return. I replaced the middle 12au7 with two different ones from the stash, and it was present with both of them.
This makes me think it is in the circuitry for that middle tube.
The sound is in time, and is basically BEEP two three four, BEEP two three four. It is a quiet sound, but is definitely there and is very distracting. Probably at about 10khz or so. the level is constant regardless of where the preamp is set.
I am thinking the timing it has is a clue. Oscillation? Could it be a stage of the cap can going bad?
Thanks for the help. This setup sounds amazing and I need it to be completely silent again ASAP! I cant go back!!!