I'd best describe the sound as a loud whirring. It happens as soon as I turn my PC on, and just continues no matter what I'm doing. I tried reverting back to Windows 8.1, but the problem followed me....

I don't know. I'm pretty much a novice with computers. When I bought the computer it had the option to automatically upgrade to Windows 10. I never envisaged encountering this kind of problem....

Ever since upgrading from Windows 8.1 to Windows 10, my PC fan runs constantly from startup. The only thing that stops it is booting into UEFI Diagnostics and then exiting. The fan starts to run...