Random sound every 15 min - what is it!?

I just bought an iMac and am loving it. I've had it for about 3 weeks now and ever since ive installed office for mac and set up time machine ive been getting this random sound clip every 15 or 20 minutes. I dont know how to descibe it other than it sounds like someone plucking a violin or cello string. What program is causing this sound and how can i stop it? its driving me nuts.

i know that its not new mail arriving, its a different sound than that. and it is different from the system alert sound.

thanks for any help

Are you positive it isn't new mail? Entourage makes a sound similar to what you are describing when new mail arrives.

open up activity monitor 10 minutes after the sound comes on (it's in Utilities)

Try to see what process comes alive as that sound plays

Open up Mail.
Click "Get Mail"
It should make a sound - is this the sound you mean?

I found it. In entourage it was the "no new mail" notification. Thanks for the help! I just turned the notification off.