"Whistling Noise Heard from the Center Fresh-Air Vent During Heating. A whistling noise is heard from the center fresh-air vents. The noise can be heard during the warm up phase (3 to 10 mins) after the heating system is first activated, with low outside temperatures and the center vents stratification adjustment wheel is turned to the ""red"" position. Note: This is independent of the air distribution setting, but a lower fan speed is required. CAUSE Unfavorable air flow against the mixing flaps in the upper distribution housing"