I don't know a lot about electronic parts so i might be very wrong. The board is an Asrock Z370 Extreme4 if that helps, the keyboard is an Ergodox planck EZ . Unfortunately i have nothing at home to make measure it myself. In another forum, someone posted his power draw is 1.5W at 50% backlight. That's little for USB.
The sound i hear is high pitched, and loud, close to the upper limit of audible noise for my no longer brand new ears. It is not coming from a speaker. I have difficulties to localize the source. I also cannot easily prepare a setup to show the problem. Right now there is this sound but it might disappear later. I'm afraid that taking the PC apart i may not be able to find any noise, as i went through this a month ago and nothing. It was there for a day, then disappeared forever, until today.
So you are saying this sound may not mean something harmful to the hardware?
It's coil whine. It's completely harmless other than being annoying. I have gone through multiple motherboards without a fix and other people on other forums have done so without improvement. Listen to the back of the PC ports, does it make a noise there?
What I would suggest for testing purposes is to set a manual OC with a fixed voltage in BIOS and see if the problem persists.