I think it's the PSU because when the noise starts, there's more air coming out of the back of the computer; where the PSU is. It's possible that...
Specs: ASUS M2N-E motherboard 2046MB RAM AMD Athlon64 x2 5200+ ~2.7GHz ATI Radeon 4850 (up to date driver) OCZ StealthXStream 600W PSU Windows...
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