What Causes Tube Noise at Start-up?

I have a VAC PA100/100. When I turn on the amp, as it warms up, I get a static crackly sound (like a dirty volume knob sound) every 10-15 seconds from my speakers.

Two of the four bias lamps, which are nice and green (good) when the amp is warmed up, start off red and you can see them flcker a bit along with the noise. When the amp gets warmed up (about 2-3 minutes) the two bias lamps go green, and the noise stops. This has happened since I replaced my input tubes with NOS Mullards.

The Bias adjust is for the bias on the KT-88 driver tubes, not the input tubes, so why the bias drift during warm-up on the two lamps?

The noise is not very loud (BTW, this happens even with pre-amp off) and I don't think anything is being harmed but I thought I would poll the experts here for thoughts on the matter. Thanks!
If it only happens with the Mullards, then it is probably a grid short in the tubes. Once the tubes warm up, the short clears itself. One or more tubes are probably worn. A tube tester will confirm a grid short.
Are these the CV-4003 or CV-4004? These are mid 1980's JUNK! Get rid of them.
try and switch the red bias light kt88s to the green and see if the bias follows the switch...if it does you need 2 new tubes the kt88...i own a vac pa 35/35....
4003. Tested fine from Upscale. They work nicely once warm.
I'll say it again, they're junk...get rid of them!
Could be a short in the tubes, could be a loose connection in the tube socket, or it even could be metal fatigue, in the tube itself, which doesn't stabilize, till the metal is heated thoroughly. Truthfully, there are so many variables, that if it is not presenting a problem, just accepting it, is probably best. The most vulnerable time for a tube, is at startup, when it begins to conduct.