I have a 2004 VW Golf Variant. There's no problem, I'm just curious.
Just after I have opened the driver's side door, a small motor runs for about one second, then stops. This also happens if I stay seated after stopping the car, and then open the door after some time has passed.
I am certain that it is not related to the door opening/locking mechanism. Perhaps it could be the fuel pump pre-charging the engine (if it even does that...)? While and after starting the engine, I can't hear that sound (but that may be because it's being drowned out).