Clicking Sound

I've noticed that sometimes when i type things, I hear a faint chirping from my computer. It sounds like a little electronic beep or something, when I hit my keys. It doesn't always do this, however. I was just curious what that was. Something with my hard drive?

IIRC it has to do with the HDD, but is not bad. My wife's iMac used to do it with 10.3 but stopped when I upgraded her to 10.4...