Dog-whistle like sound coming from iMac?

All of a sudden, I wake up on Monday (today is Friday) and my iMac is emitting a very high-frequency noise coming from inside. I can only describe the noise to what a dog whistle sounds like. Very even, constant, high-pitched noise. No clicking, no motor-like noises.

It's exactly like when your ears ring - but unfortunately, it never goes away.

It's only an annoyance but I would like to know if there is anything I can do.

Here's what I concluded thus far:

• It's not the fans running on high. That happened to me not too long ago, so I know what it sounds like.
• I don't think it has anything to do with the screen/LEDs because:
  • The noise does not change at all when I adjust the screen's brightness level.
  • The noise remains when the display goes off. I changed the energy saving options to make the display sleep before the computer and the noise remained constant.
• I opened up Photo Booth and that had no effect either.
• The sound is coming from the right side of the iMac because when I put a card in front of the space in between the computer and the desk and slide it left to right, the sound eases when the card is in front of the right side.

I am aware that iMacs make a light buzz from CPU/HD sounds and stuff. I have had two iMacs since 2007. But logically, you would think that after 9 years I wouldn't notice any "regular sounds". I am noticing this sound which makes me think it's new.

(Am I just losing my mind and hearing ringing in my head??? Cuz I would believe that too LOL)

Any help would help! Thank you!