Door Lock Activates a Pump/Motor

About 2 months ago I noticed that when I unlocked my car, a small motor started running under the rear of the car. This is a 97 Passat GLX Wagon VR6 with 115K miles on it. The motor makes a buzzing sound like a fuel pump. This happens nearly every time I unlock the car unless it has only been sitting for a short time. Outside temperature doesn't seem to influence it. Lately I've noticed that this motor doesn't kick in unless I give the key a 2nd twist to unlock all doors. If I only unlock the driver's door, everything is fine. When this motor does kick in, it continues to run for maybe maximum 10 or 15 seconds after the car has started. If I don't start the car, the motor will quit after about 30 seconds, and when it stops I can hear a flapping sound from the rear like some flexible line is reacting to the change in pressure or whatever.

I know if I bring this to a mechanic they won't be able to figure it out. It's just too weird. Does anyone out there know if there's a possibility that in the electrical system on these cars something could get crossed between the electric locking system and this motor I'm hearing? I'm using the term fuel pump loosely because since I can hear this motor stop after the car has been started, it must not really be the actual fuel pump. Maybe it's some other motor back there? Any thoughts on what is causing this?

The door locks are vacuum operated & that pump is what opens & closes the locks when you turn the key in the door. You may have a small leak if the pump runs long or the other doors's locks are slow to move.

Thanks guy, I'm glad to hear it's just the locking system and not some kind of electronic riddle to solve.