I'm guessing it's the hard drive parking/unparking--that will happen periodically (when the drive sleeps for power savings) and when you put it to sleep (as it writes data to the drive then turns it off). It varies a little based on the particular drive model, but usually sounds like a soft click or "ker-chuk" sound.
Easy test: Put your ear down by the computer and close the lid. You should hear a soft chattering sound while the computer backs up the RAM to disk in case it gets power cut, and then a few seconds later the'll be a click and it will become completely silent. If that last click is what you're hearing, it's the hard drive, and it's almost certainly normal.
If it's not that, hard to say. Another possibility is the speakers waking from a sleep state (which can happen even when the computer is awake if there's no sound for a while), which can make a soft pop (or at least, so I've heard--my MPB doesn't do that).
