Hmm....If you see an egg next to it, I'd be worried...
Are you using Adium? I know that Adium (because of it's cute little duck icon) honks when you send or receive IM messages. I have this disabled in favor of using Growl for IM notifications, but I don't know if Growl supports Leopard yet. Still, you can disable or change the honking quack sound to something else in Adium.