Hard to say how serious it is, but you should check into it.
Might just be a loose connection or bad bios setting. Could be the motherboard is on the way out.
A decent explaination here:
http://forums.techguy.org/hardware/98502-motherboard-system-timer.htmlThis one has a link on how to Troubleshoot the motherboard.
http://www.pcguide.com/ts/x/sys/beep/amiB4-c.html