I don't think it is a fan...I've opened the case and have tried to pinpoint it, it seems it could be coming from the lower part where the PSU is, but I'm not sure. Also, I'm not sure if this is related...but when I started my PC i noticed at the one of the screens where is shows the mobo brand is says 3 cores activated, but I have a 955 Phenom, I seem to remember it saying 4 activated but I could be wrong, but then when I ran cpu-z it lists the name as the x3 phenom but under specfication it says x4 955 and at teh bottom is says 3 next to cores and threads.