beeping out of park

On our '03 LT, we get a series of beeps every now and then when we shift out of park just after starting. The beeping is the same kind of beep when I hit the driver recall (1 or 2), but it beeps about 5 times in a row (not the chiming for the door, key, or lights). I couldn't find anything in the manual on this. There are no lights (like check engine) that come on during this time. And when it occurs, nothing else is happening. Plus, this doesn't happen all the time, and I still have yet to find a common instance that this happens... any ideas?