The fact that you heard a pop and then things were better for a while leads me to believe it may be a static electricity issue of some kind. The pop would be the discharge and since the energy had been discharged the problem went away for a while, the time it took to build the energy back up again. I wonder why just the left channel. Are you in a dry climate? If so, you might try a humidifier in the room to see if it has any effect. The electricity could be coming from other sources but I know if a room is dry the static is much worse. Static and capacitance are the only thing I can think of that take a time to build and would discharge upon touch or disconnecting.

PS Audio carry some outlet products that isolate digital and analog sources. They even have some outlets that don't limit current for power amps.

I just remembered and looked up and sure enough you are in FL so maybe you aren't in a dry climate but it still sounds like static.

You have a nice system hope you can solve the mystery.