If you’re running Windows XP, it makes these sounds for a few reasons. One is that new updates have been downloaded and are ready to install (check the tray, bottom right hand corner of your screen), that new hardware is has been found and it’s looking for drivers, or that an externally connected hardware device (firewire hard drive, USB card reader, maybe a printer) has been disconnected or reconnected.
Generally, it’s not serious, just something the OS thinks you should know. If it really bothers you, I guess you could assign no sound to those kinds of events, in control panel | sounds.
If you’re not running windows XP, I have no idea, tho I think MacOS does a similar thing.