Am I crazy or is it my dac?


Recently my system hasd eveloped a crackling sound in the background when playing tunes thru the Schitt Gungnir Multibit and I'm trying to figure out the source or whether I've just gone nuts

When I first fire up the system, everything sounds fine, then after 30 mins or so when I'm playing music, while the tunes sound fine, a background crackle happens, like a scratchy old recording - though I'm playing files which previously sounded fine.

If I pause play, the sound stops.

Ideas?
Is it the dac? 
Anyone experience something similar?

Thanks in advance,

System
Aurender N100H
Schitt Gungnir Multibit dac
Belles 21a preamp
Primaluna Prologue 7 amps
Martin Logan Ethos 

cdc2
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It was in just the left channel.

Turns out it wasn't a tube, but the connects. When I unplugged every connector and power plug from the doc, and replugged them all in. Problem solved.

Now onto to diagnosing the 60 cycle hum from the amp in the right channel.
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I think majority have so far concurred it is likely a tube especially if you find it is just in one channel.

Good luck!
Thanks for all the advice. Now for the testing phase to see if:
A.  Is one channel or both
B.  A connection issue
C.  Issue with router being too close to system
D. Dying tube (with 4 power tubes in each to test)
I’ll bet money that it is a tube in the PrimaLuna .I had the same exact problem before.
It was an output tube /power tube. If it is in one channel vs the other.
Start by switch each output tube one by one from left amp to right amp, once you know which amp Left or Right. when it switches to the other channel,that’s the culprit.
Always give the amps 10-15 minutes of cool down before switching them back on, once you have swapped the tubes
If its in one channel its likely a tube if its in both take a look at what's around your system. Anything that can create noise like a modem/router or anything? I had a similar issue turned out to be my wireless gateway generating RFI.
Could it be a tube ( or several) on the way out. Which might account for why it is fine until everything warms through a bit.
Just a thought....
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