Hooting or Whistling noise. Owners of some 2005-06 Ford Expeditions and Lincoln Navigators with dual a/c systems may hear an irritating hooting or whistling type sound from the rear of the passenger compartment.
According to Ford, the noise occurs most frequently when the vehicle is being driven with the front a/c on, the interior cooled, the rear a/c off and the blower motor speed set its lowest setting. A defective rear expansion valve has been identified as the likely source of the noise. Installing a new-design TXV (Part No. 6LiZ-19849-B) into the re3ar HVAC case should put the problem to rest. Should take about two hours for the part to be replaced.