iMac strange alarm sound?

Not sure if this even came from my imac but it sounded like it did (i have a lot of other electronic stuff on my desk) but has anyone ever heard an iMac G5 make a 3 tone alarm sound (that repeats a total of two times)!? It lasted for about 4 seconds. Maybe I'm going insane

heh, it was my iPod making the noise, sorry not sure why it does that at midnight.