Computer keeps making bell sound

[tacogir]

My computer keeps making the oddest sound. I'll be doing something and it will make a bell sound. It sounds like an actual bell. I first noticed it when playing world of warcraft and thought it might be one of my addons, auctioneer, because it sounds almost exactly like the auction bell that sounds when you enter the auctionhouse, except ringing once instead of twice. But then my brother complained that it kept doing it during the night. I've had it happen several other times as well. I've gone through my sounds on the control panel and there is nothing like it (I'm using the default XP sound scheme). I'm starting to worried, is there something wrong with my PC? I'm running XP SP2.