Computer noise (Not fans)

Not too long ago, my computer started making this abnormal humming noise, something akin to the gentle hum one would normally get when starting up their desktop, except louder and with a sort of mechanical buzz to it.
Do forgive the lack of a better description.

The odd thing about this noise is that it only happens when my computer is booting up or idle. When I start playing a graphically intensive game, for example, the noise gradually softens until it completely goes away. Similarly, when I stop playing the game, the noise slowly returns.
This led me to think that perhaps voltage could be an issue, that perhaps idle voltages might be too low and hence cause a noise, so I disabled the C3, C4, etc power states but to no avail.

I know for a fact that the cooling fans are not the ones causing this noise, as I have disabled them yet the sound still remains, nor is it due to the power supply fan.

Any help would be greatly appreciated! I find many posts regarding sounds appearing when playing games, but have had little luck when the situation is reversed.

If it helps, my CPU is an i7 4790K, and GPU is a GTX 980Ti

Thank you!