Air Vent flap makes noise when starting/stopping air conditioner

When I start my car and then when I turn on the air conditioner, the valve that may open or close to take the air in does not seem to work as required and makes noise (which sounds like tak, tak, tak) for about thirty seconds to a minute and then shuts down. Same thing when I turn off the air conditioner. Can you advise what is the problem and what needs to be done to fix it and approximate cost? Thanks.