Airconditioning valve noise

[Fly Car]

With my Model S 2013 model, I have a noice (thud) every 20 sec or so and it sounds like someone is closing a car door near by. After some investigation it appeared originating from the ventilation system and stops when air conditioning is switched off. Anyone any ideas if this is a problem with other cars and how can it be solved. Hate to go to Tesla as the changed my xenon head lights and charged me 600 Euro's