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