allthatglitters,
Open the case and listen for the noise. There are fans usually on the front and back of the case, Video Card, Processor, Power Supply and sometimes on the Northbridge of the Motherboard itself.
Also if the inside of your case is full of dust, then you might need to hit all the computer with compressed air. (Shoot everything, All fans, Heatsinks, Power Supply etc.)
Let us know which fan is causing the problem.
Take Care,
Richard