Hi there AlexIrwin,
Unfortunately, many things could cause this high pitched noise.
One thing you can try is to just detach the PSU, attach it again and see if the issue persists. You can try using your old PSU as well.
Are you sure the sound comes from the MOBO/CPU? Open up the case and try to localize the sound.(HDD, fans, GPU)
What do you mean by "wouldn't boot up"? What does the screen show?
Let me know how this goes,