If it's the same contactor sound your PC makes when booting up and shutting down, it's most likely your PSU. What is the model of your PSU? How old is it?

It's most likely originating from a relay switching inside your PSU. It's perfectly normal - my Be Quiet unit (slightly ironic, I know) does it and I've never had any issues.

That's probably the physical power switch actually clicking off and on. Did it just start happening?

Sounds like the overcurrent protection is tripping on the PSU

Oh, I forgot to make any update to this case.

The culprit was Windows 10.

I don't know why, or how, but every other installed OS works just fine.

Asrock BIOS update has been wonky though, it got from stable EXPO to not handling EXPO at all.

Due to heat, materials can bend. If a material was under alot of tension already before, it can sometimes pop (click sound) to release it. Just like poping a pimple.

Depends on what exactly it is and how exactly it is poping it might be a problem but usually you should be fine.

Could be a fan starting up and hitting something like a loose wire.