Kitchen extractor hood vent keeps flapping?

I have installed a new hood above the cooker and vented it outside with a covered down faced flap which should only open when the hood is being used,however it is continually flapping outside and it echo's down the pipe to the inside.
When i open a window or external door to the kitchen it stop the flapping but as soon as i close it again it starts off flapping again.WHY????
And what can i do to stop it please .