Does this happen only during warm-up; perhaps within the first half hour only?
Could be just a bad connection developing between tube pins and socket connection. Try removing and re-seating the tubes, power and small tubes. If you can identify the specific tube, and you might do this by rotating tubes from channel to channel, you can also try to tighten the contacts in the power tube sockets if the former doesn't work.
Thanks to all for the suggestions. I will re-check the bias, although on my ARC amp this is a real chore...:0(
The driver tubes, as well as the power tubes, are all fairly new with less than 50 hours on all of them. What I am wandering is if my JJ driver tubes are going noisy? JJ's seem to be infamous for this problem although I do like the sound of these.
Yes. It could be JJ's. I've had noisy ones. But as often as not with newer tubes it was a seating/contact problem. One of the reasons I suggested that as a probable issue for you was the appearance of the noise during warm up but not continually thereafter. Metal shrinks when cold and expands when hot. When hot the tubes pins would be making better contact.