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?
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..