Blue glow on preamp tubes - what is normal?

I have an intermittent scratching noise on the left channel of my tube integrated that only occurs during the first 15 minutes or so when the amp is warming up. Not knowing if this was normal, I attempted to isolate the problem tube by replacing each tube at a time.

I first suspected the preamp tubes so I swapped out the left channel 12AU7. I am noticing that right 12AU7 has a blue glow on both sides of the glass, whereas the new 12AU7 has no glow on one side and barely has any glow at all on the other. The 2 12AX7 tubes have no blue glow. Is the glow any indicator of tube life? When replacing one preamp tube, should I be replacing the other tube as well?

I understand that the blue glow is common on a lot of output tubes, but not sure about driver tubes. BTW, the noise during warmup is still there so it's probably the power tubes that need to be replaced.
I would check the base of each tube for any indication of arc.

The blue glow could be nothing at all or the tube that is causing the problem. If the blue glow is cloudy looking, I would replace. If the blue glow is more of a slime, I would be less inclined to replace.

If nothing else, checking the base of each tube and reinserting them in their sockets could clean the pins enough to cure a contact problem.
It is possible that there is an intermittent connection at one of the pins, that is making better contact due to thermal expansion, once the tube and socket warm up.