Internal PC alarm??!!!

Hi after using my computer for about 2hrs a alarm sound starts from inside the base unit (not the speakers) and doesnt stop unless i turn it off? is this something to do with over heating? im using a ABIT motherboard with Intel P4 2.81 cpu which also has a large intel fan attached which i can here when the computer is running, so im sure its not that? any suggestion? it sounds like a lound digital alarm clock if that means anything. and i dont get any warnings or messages on the screen when it starts??? help!!!!!