Ive got a post on the cases part of the forum, but when i started the thread i thought my problem lied with the case, but now that i know its the Power Supply Unit, i would like to see what people think. The problems are:

Screeching Noise - which spools up and down slowly, occurs at even cool temps(even when performing non demanding tasks), when side panel taken off, the noise goes away, when put back on it returns slowly

When PC is off, but power supply still switched on, a buzzing noise is present

The pc sometimes turns on by itself, wakes me up

Could just be the hard drive, but i hear faint crackling noises from the supply, even just performing non demanding tasks

Yes definetly sounds like a bad power supply. Dont have it plugged in while this is happening. Dont run the pc until you have replaced the power supply.

Its been like this for some time now, only recently has it started powering up the pc on its own, what could happen?

your power supply could fry your motherboard, or even worse, explode (literally) and cause fire, destroy your entire pc, and potentially cause bodily harm. And i'm not joking

Hey, i have just recieved my pc back from the shop, who said they could find no fault and no noises, but i am positive the were there and i have people who have heard it, the only thing they said is that it could be the electricity in my house that causes the problems, which i find hard to believe. Since starting the pc, i have heard the noise at a lower pitch spooling up and down, not as loud either, but i am still concerned. could you please give me further input/advice.

one way or the other, its likely your power supply. I recommend replacing it, to be on the safe side. There is no real way to fix a bad power supply unfortunately.

EDIT> Im assuming you still hear the buzzing sounds and the other sounsd. otherwise, it could just be the fan. but from what you told me before, it could be both.