Random Beeping As I Game

Hi, whenever I play any intensive, graphics heavy games like Assassin's Creed or Hellgate London, my computer will start to randomly beep about five-ten minutes into it. It'll beep once to start with, then a short while later (maybe 30 seconds?) it'll beep twice, then after that (again, maybe 30 seconds) it'll beep continuously, fairly often and long. It seems to only happen when I'm doing things that are fairly demanding, when I'm just browsing the internet, typing Word documents, etc. no beeping occurs.

I don't think it's due to overheating, I measured the temperature of my graphics card during one such beeping episode and it was only at 51 degrees celcius, which I read was a perfectly fine temperature.

I'd really like to find out what it could be, I don't want my computer to break. My specifications are below.

OS: Windows Vista Ultimate 32-bit Service Pack 2
Processors: Intel Core 2 Duo CPU E8500 @ 3.16GHz
Memory (RAM): 4.00 GB
Video Card: ATI Radeon HD 4870

If you need any more information, please just ask.

Thank you in advance for your help.