Air vents

If that occurs during acceleration, look for a leaking check valve in the vacuum hose feeding the heater controls. If it occurs when cruising at a steady speed, suspect a leak in that vacuum hose. Vacuum-operated systems are spring-loaded to default to the defrost position when there's a problem with the vacuum supply. Your feet might freeze but for safety, your windshield will be clear.

In the l/f fender there is a canister in front of the wheel. You see a loose vacume hose thats needs to be reattached. Look from the bottom. Gl