Mystery notification sound on Mac - please identify!

I have a sound that plays seemingly at random on my MacBook Air but I have no idea what the notification is trying to alert me to. Nothing pops up or otherwise clues me into what is responsible.

I've attached the audio capture to see if anyone can identify:

https://drive.google.com/file/d/0Bwvx3nRHv9ESdWY4ZFhlNTk1aDg/view?usp=sharing