car makes a flapping sound . something to do with heater door?

Hello, where and when do you hear the flapping sound?

So it seems you have had some-one at least check this out for you as you stated “ something to do with heater door” .. Now what do you want to know here exactly - how to do the necessary repairs? The thing that’s the most likely suspect is called an actuator. This is not a task the average DIYer should attempt.. Really do not understand your question.