Weird "ding dong"/shooting star notification sound

I'm on a Macbook running 10.5.8. Here's my list of running programs. I keep getting this random notification sound like a shooting star, a little "ding dong." I'm not getting a new email, no visual notification, and it's no at particular times (I got it a 8:15 and now 8:39, for example). Any idea what this is? It's creeping me out.

Are you running either one of these applications growl or adium?

Click on the underlined text above, you can see all the programs I'm running.

In Safari did you get any Extensions lately.

osxx said:

In Safari did you get any Extensions lately.

Click to expand...

Unless it came with an automatic update, no. I only use Safari to keep Facebook open for messenger. I don't do any browsing with it and I haven't downloaded any extensions for it.

Still no idea.