They can see the flap without taking the whole dash apart. Not sure how... maybe by removing the panel with the air bag switch. They told me they could see the flap broken before they even started the job. I think it got bad enough to where it just broke and it was very noticeable.

Some people that hear the noise have told me they cleaned junk out of the blower and it went away. I would do that first (like I did). If that doesn't help, go for the flap. I am not sure how it works but I think it opens and closes to allow outside air in or to recirculate the inside air.