This happens quite a lot! I asked my older brother about it (who's had a couple of GTi's and re-built one!) and he said it's just the "flaps" inside? If they're going, does that explain that when I'm on the motorway, inside my car gets boiling hot, obviously where the engine is chucked heat back and it's coming through the vents, because there's nothing there to stop it doing so!
It doesn't matter to me at the moment, but just checking its nothing to worry about! Prob a common one?