Argh!
It's the side fan in connection with the side panel of my Corsair 200R case. When disconnecting it, the PC sounds great. When I move the bottom of the fan away from the case, the hum disappears. As I move the bottom of the fan closer to the side panel, the "hum" comes back. I've tried different screws and that's not helped. I've tried them screwed in loosely, no different. If I screw them too tight, the screws actually go through the rubber and the fan is loose on the inside of the panel.
Looks like the problem might actually be with the design of the side panel of the 200R unfortunately :/