If it keeps doing it and it sounds constant and nonstop, it's usually a key that's stuck. I spent 3 hours trouble shooting this once only to find the the tilda key was stuck down on my keyboard. Just try a different keyboard.
I just repaired a similar system, and messed up with the RAM. An unfortunate move caused a static shock to zap the DDR memory.... stuck it back in the machine, and similar beeping problem ensued. Machine would shut back off in my case. Was trying 400MHz DDR 1GB sticks earlier and had similar beeping problem. Putting in old 128MB DDR 266Mhz solved that problem.