tube noise - 2 questions

Q1: Three months ago, I installed two matching NOS 6DJ8’s in my preamp. Sounded great up until a week ago. Barely audible tube rush on startup - and then totally quiet. Three months later, one started making crackling noises. Only intermittently.  It will crackle for a few minutes - then all is well again for days. But, this repeats every so often. Does this intermittent problem mean that tube is soon to be history?

Q2: With a different preamp, a NOS 12AU7 had crackling & popping noises from the get-go. The noise never went away - even after 100 hours of burn-in. I tried the same tube in my DAC, zero noise - ever. Why might that be?
I've experienced similar sounds with tubes. If it goes away, or is intermittent, you should be ok. The tube rush sound will only get worse over time, but usually takes a long time to get there. If you can hear the tube rush from say, 2 to 3 feet from the speaker, not much of a problem. If you start hearing it at the listener's position, it might be time for replacements. IMO
Any issue with a tube in my tubed hifi rig or guitar amps becomes grounds for dismissal. Having use tubes for over 50 years I've been amazed at how rare issues with bad tubes have been, but when they have become noisy or lose their sonic charms in any manner, they're OUTTA HERE.

Q1: the tube has an intermittant problem. Giving it a good whack on the side might shut it up for a while. Eventually that will get old- just change it out.
Q2: This depends on the gain of the circuit. In a DAC, the gain is relatively low, but if you use that same tube in the front end of a phono section it might be quite noisy. If its not a problem further upstream or in the DAC, that's probably where it should stay.
Like atmasphere mentioned  change the tube. Clean the pins and the tube socket.  If using a new tube and you still hear the noise and after wiggling the tube the noise comes and goes then probably replace the tube socket.  Happy Listening.
@atmasphere        It worked!   I tapped the noisy NOS tube with a pencil.    The intemittent crackling is gone - so far.    Thx for the inexpensive, low-tech solution.

You can count on it coming back again... Its giving you plenty of warning that you need to find another tube.
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@ebm   I compliment you on adding punctuation to your posts. It’s an improvement. Spelling will be your next great challenge. Perhaps in time, your posts will actually be helpful or informative.