94 f150 4x4 pick up

Try cleaning or replacing the filters there may be one on the passenger interior somewhere on the glovebox but i dont know as far as 1994 goes back, maybe air intake vents covered with debris at the bottom of your windshield? Could be preventing airflow and not be as bad when the air is essentially 55mph

Try cleaning or replacing the filters there may be one on the passenger interior somewhere on the glovebox but i dont know as far as 1994 goes back, maybe air intake vents covered with debris at the bottom of your windshield? Could be preventing airflow and not be as bad when the air is essentially 55mph. Could also be at the top left corner of your engine bay insertedhorizontally in the area of the passenger dash accessible from under the hood.