I wouldn't be surprised if it was the PSU. I bought a used 700ish watt PSU and it made a really loud sound. The annoying thing was, you can't tell whether it's the PSU or some other fan on the computer since all fans make that same sound! I only figured out it was the PSU when it failed on me. As Mellors says, it most probably isn't the CPU fan, it's either one of the case fans, the graphics card fan, or the PSU. Test each fan and see if any one of them is the culprit. If you have another graphics card lying around, plug it in and see if it's your graphics card that's causing the problem. If it isn't either the graphics card or the case fans, it's definatly the PSU.
When buying a PSU, make sure you get a good one. Antec, Coolermaster, Enermax, Thermaltake, Zalman, OZC, and Corsair are good brands. I've had PSU problems in the past, and let me tell you, getting a cheap one isn't worth the time and money. Within a few months, the cheap PSU will go bad, and you'll have to either deal with RMA'ing or just buying a new, reliable PSU. Don't make the same mistake I did. Do it right the first time and buy a good PSU.