Are there two flaps? The reason I ask is that my car sat for so long that they must have been sticking. Then one day I after about 60 seconds I heard the thump and assumed it was some sort of heater flap. I do not have heat on passenger and posted about it. It was deemed that I may have a stuck solenoid but I still need to address it. Maybe I have a flapper problem.
1952 MG TD
1963 MGB
1967 MGB GT Special
1974 MGB Roadster
1991 Ford Ranger
1997 BMW 528I
2000 Harley Davidson Softail
2004 Ford Expedition