Leafs blowing inside
Leaves fall into the cowl vent and then get pushed into the interior vents. Sometimes they get stuck in the heater or A/C box, but they can just come in through the vent boxes. If the car isn't currently being stored outside under a tree, it means the cowl is full of leaves. You can pull the vent boxes on either end and clean it out as best you can.
This is a non-AC car, but I think your '71 has all the same vent boxes. I dropped the vents and the box, reached up into the cowl, and pulled down all the stuff you see here from inside the cowl.
vent boxes
cowl space that's probably packed full of leaves...