When I’m trying to find the source of a sound, I find touching things to help. Particularly if you can touch a part, then start the noise, then stop the noise. Although if the pitch is high enough, you might not be able to feel it. I’ve seen posts about pretty much every part going bad, so you will want to narrow it down to a single part.
If it is the power supply, it could be a couple of things. There have been several posts about various noises and repairs people have made to the power supply.