It could also be a noisy northbridge (NB) fan. But if your motherboard has a passive NB fan, then it's probably the graphics card fan as rdave13 suggests. What's your graphics card make & model please?
What's the make & model number of your motherboard? Use SIW click here (standalone version) if you don't know. Run the program & look under Hardware > Motherboard. G
Thanks for your help it turned out to be the fan on the grahics card. I have cleaned it and used some sewing machine oil on it and it is much better. I think I can live with the remaining sound it makes.
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