1994 Volvo 850 Flapping noise under dash with AC or heater on when speed above 45 mph!

The fresh air or recirculation door which is above the heater box could have a loose cable or be broken. You will have to remove lower A/C cover on the passenger side and then the blower motor to see if you can reach it and check if cable is properly held in place by actuating it. Ther should be a clip that holds the cable, outer part of cable, so it stays in place to keep the inner cable from moving and to pporperly actuate door. Also check to see of hinges are broken. You could have vacuum actuated door so check for a vacuum leak and make sure the hose is connected in that case. Then with the car on put the door on recycled air and turn off and quickly check the dorr to see if vaccuum pot is holding door closed. You can leave cover and blower motor out for this. Trun blower switch of to be safe.