I have given up trying to achieve a quiet PC.   Even at 20% fan speed I can still hear this subtle SHHHHH SHHHHH noise of the EVGA fans.   Sounds like a very faint plastic on plastic rubbing noise, but it's there and prevents me from running the fans at a low speed at idle, which in turn forces me to use the "adaptive" power mode otherwise idle temps go from 40C to 60C (with web browser open, which stops it from downclocking). 
 
I also noticed my Noctua NF-F12 CPU fan has the same kind of thing going on as it has this ~120hz sine wave like hum at 1000RPM but not at 1200RPM.   I think it's the way the air is interacting with the heatsink.  Room acoustics may be to blame as well if I move around to different parts of the room, it gets louder/quieter.  The EVGA @ 35-41% is still audible from anywhere in the room.    But even just the positioning of my PC case in the room has a significant impact on overall fan noise.   If I have it to the right of me I can hear the case fans humming, to the left and lower frequency hum is gone.   If I remove the side case panel, that also lessens the hum too which is weird because you would think it should be the opposite.   I guess sound echoes around inside the case and amplifies (standing waves?).