Tube Diagnosis

I am a bit limited in my experience with Tube quirks so looking for a bit of insight.

With no music playing I noticed a small amount of noise coming from the left channel of my system a couple weeks ago.

I isolated it to my preamp (Aesthetix Calypso - 12AX7 gain and a 6922 output in each channel) and then further to the left channel output tube. I did this by swapping the left and right output tubes and the noise moved with the tube.

I am running used tubes with a reasonably low but unknown amount of hours, all was silent until a couple weeks ago. Telefunken ECC803S for Gain and amperex PQ white 6922's for Output.

The 'noise' is a low level crackle that seems random. i.e. crackle crackle---silence for a random amount of seconds--crackle and so on. It is independent of volume and does not change if the tube is lightly tapped! The noise is low as I mentioned but can be heard during silent sections of music and is distracting enough for me to buy new tubes to get rid of it. The only other thing I have noticed is that the noise is a bit more pronounced when I first turn on the system, after it warms up things mellow but the noise does not go away.

I have experience with tube rush and what I would call microphonic noise - tube noise that increases with volume and responds to tapping of the tube. This is something different.

So, is this the sound of a Tube that is dying or just a tube that went noisy and will now stay that way?

Thanks