Macbook Pro 13-long beep on startup?

Hello,
Something happened again tonight. When I started up my MBP, it made a long single beep noise, then showed the mac logo with a progress bar below it, then flashed the screen, and went to the regular startup with the mac logo.

This is the second time it has done this. The MBP is not very old. I bought it in the last of September and hardly ever use it.

I also ran an AHT, and it said that no errors were found.

It's a:
MBP 7,1 13"
Snow Leopard 10.6.5
C2D 2.4
4GB RAM
completely stock internals

Any suggestions?

First thing I would check is make sure the RAM is seated properly. Don't know why it would randomly come unseated, but thats what I would check initially.