In answer to several questions, it was network sleep. And I don’t use HDMI or USB with the Nova.
I tried moving the speaker cable and the first time I turned it on there was no pop. But the next time it was back (relatively soft).
I had already tried unplugging everything on the rear except the speaker cables and power cord, but I still heard a pop.
By the way, I usually get one on turn off, but that is very soft.