2005 Expedition EB making noise in dash when heater on

Ok, now that we can see the end of summer from here, my 2005 Expedition Eddie Bauer is making a banging noise in dash when I turn the heater on. I have the
dual temperature controls for the front (driver's side and passenger).
I figure that the flap between the heater and the air conditioning coil on one side (or both) has broken loose from the motor and the motor is constantly trying to adjust it. But all that happens is the banging noise. But, my heater does work fine.
Expensive to fix? With 145K miles, I am not interested in dropping much cash on this fix.
Thanks, Lynn
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