OKAY! THAT'S IT! What IS THAT NOISE?

That is the 5,000th time my computer has made that noise.
Here’s the story:
I threw a barring in my MacBook Air and took it to the Apple Store for a warrenty repair (which I hate doing. I usually fix all my own stuff.) Anyway, I get it back, everything is fixed and pretty, I bring it home, spin up the fans and everything is working okay. But then about an hour later I am here, on Fluther, writing a respounce, when my computer made this strange, zipp-ping-ping-beep noise. Like some kind of sensor recored something, then beeped to let me know. The Mac has never done this before, on any of my computers, and I am wondering what it is, or if you guys have ever had anything like this happen?
The Apple Store says that there is nothing wrong, and I can’t recreate it to show them, not without sitting around typing for hours on end. The Geniuses say that it is probably one of my programs… I know it is not because the noise comes from the back of the Air. So far nothing seems to be broken by this, and my CPU and RAM don’t report any kind of errors or anomalies, so I am not concerned, but I hate that damn noise, it get annoying… VERY annoying.