The dealer replaced the A/C evaporator coil to fix my complaint about the air ducts making a whistling noise, the fins that the air passes through have a different design on the new evaporator coil, this did stop the air noise. The replacement kit included a new blower housing and a condensate pan with a longer drain, the installer told me it was about a 1/4" longer, I hope this also fixed by A/C condensate leak, I will not know until next summer. The replacement required the dash to be pulled apart, heater core to be removed, and the A/C recharged with refrigerant (big job). There is a TSB 08-9-8 MAY 08 A/C - Whistle Noise When Changing Temperature. Some 2007-2008 Expedition and Navigator vehicles built before 12/4/2007 may exhibit a whistle noise from the heater/air conditioning while changing temperature setting. If you are pre 12-04-07 and still under your 3 year 36k warranty, you might want to look into this..

 

Can you explain how the whistle sounds and when it is created? How much of a temp. change creates this condition?

 

Mine does it. I just have not taken it in yet.

 

Does it in heat only, this will make it the worse….. Set air to blow out of floor and vents and the same time, fan ½ to ¾ speed, now adjust the temperature, starting with a low setting and then start to increase slowly one or two degrees at a time, and at a given point, when it calls for more hot air, the air ducts will start to whistle..