So I kinda figure it's either the PSU or one of my hard drives, but what the issue is is that me PC first makes a click sound (as if I physically shut it down), then restarts some time after.
When it restarts it makes a bit noisier click sound, same as the power button switching does.
I have verified it's not ghosts or little green men.
I can't say for certain about Martians.
It's just weird. I'll be unplugging both HDD's today to see if it goes away.
Then one-by-one plug things in/out.
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 a Seasonic Focus PX-650, so quality PSU, but also 3 years old.
That doesn't seem that old. There's a chance it could still be under warranty, if it does turn out to be your PSU.
Oh it definitely is still in warranty, it has 10 years total warranty. Ordered a 750w model to see if it stops the shutdowns, after that I either return it or trade it from warranty.