it's definitely coming from something electronic, it's not from or through the speakers.
I would think that would make it easier to find as you may only need to have one component powered up to hear the sound. So, sometime when you can be in the room, but not listening to music, simply power up only one component for a reasonable period of time to determine if you hear the sound. If not, power it off, wait an appropriate period of time to determine if you hear it after power-off, then repeat with the next component. Worse case, it should take you less than 2-3 hours based on your description in the original post.
Good luck. Hope you solve this soon.