I have a

Gateway running Windowsxp pro, occasionally I get a very loud hum. The only thing that preceeded this hum was that I installed a new monitor (Samsung 971p) I took the cover off thinking it might be the fan but, can't find the the fan. The sound is comming from the compartment that houses the power supply. This is not a constant sound, it's intermittant. Would the fan be in the compartment with the power supply? Or could the power supply be going? Is that something I could replace myself or would I have to take it to someone for repair? Even when it is making this loud noise it proforms fine, nothing seems to be effected by it. Please help I don't want to loose my machine. Thanks