Help oh mighty PC personages!

[Pebs]

My pc is made up of castoffs from my ex's pc. I know NOTHING about it, other than it's black with a funky blue light on the front.

It makes a noise when it's been on for about 10 minutes, kind of like a anaemic ambulance which I understand means its overheating. To be honest I ignore it nothing has blown up yet and its been doing it for months. But I do notice it's getting a bit slower in it's old age poor thing. Also, I'd like to get some speakers (at the moment I have to have stuff reeeeally loud so I can hear it from some headphones which are on the desk...I cant put them on because then I wont be able to hear when Daisy starts dismembering the gerbils) but I cant have speakers cause then I'll hear that noise all the time which will drive me nuts.

My question really is what is likely to be making it make that noise? And how do I make it stop it? And dont report me for pc abuse, I weren't me guvner!