I agree with Jmcgrogan2. You probably are looking at a grounding issue (or possibly some other weird thing on the circuit).
If all was well with the Chinook for three months and you have not moved anything around in the system (because, if so, perhaps a solder joint broke loose or something like that), I'd actually look towards something that may have recently been plugged into the circuit elsewhere in the house.
Hums dont magically appear in a system that was non-humming system for 3 months. Like I said, if you moved components around, then maybe I could see it. Otherwise, you may have something else dirty on the line a few rooms over.
The hail-mary longshot is that something in the Chinook has gone bad (a tube, etc) but that is unlikely, IMO.
BTW, good luck with resolving this. My biggest pet-peeve with ANY system is noise. It drives me crazy, even if it is really slight. Sure, I may not be able to hear it with the music on, but just knowing it is there ticks me off.