Unidentified Droning Sound from PC

My PC is having occasions where this droning sound just starts, then slowly quietens down and dies out.

Recording: https://vocaroo.com/i/s0ljAAdl7xxj

It usually happens for a stretch between 30s and 2mins, and I can't identify the source.

It doesn't seem to be the Case fans, I've unplugged them 1 by 1 each time the sound occurs and it still continues after they've stopped.

Does anyone recognize where it might come from?