This may or may not be it. Might have a hose unconnected. Turn it on and get under there with a flash light.

TSB 08-9-8

05/12/08
WHISTLE NOISE FROM THE HEATER/AIR
CONDITIONING WHILE CHANGING TEMPERATURE
SETTING - BUILT ON OR BEFORE 12/4/2007

FORD:
2007-2008 Expedition

LINCOLN:
2007-2008 Navigator

ISSUE
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.

ACTION
Follow the Service Procedure steps to correct the condition.

SERVICE PROCEDURE

Confirm customer symptom by cycling the temperature door as follows:

Manual System:
1. Set blower motor speed to number 2.

2. Set mode to PNL/FLR mode (most noticeable). Whistle may be audible in Vent or DEF/FLR but to a lesser extent.

3. Set the temperature **** all the way to full hot.

4. Start backing off full hot incrementally while listening for whistles. Whistle should be audible when the **** position is roughly between 5/8 and 3/4 position.

Dual Air Temperature Control (DATC) System:
1. Manually set the blower motor speed to medium or medium high.

2. Manually set mode to PNL/FLR (most noticeable). May be audible in Vent and DEF/FLR.

3. Set the temperature set point to highest temperature 90 °F (32 °C).

4. Start reducing the temperature set point incrementally while listening for whistles. Since the blend door position is dictated by several inputs like in-car temp sensor, ambient temp sensor, and sun load sensor. The whistle can be anywhere in the range except full cold.

If the whistle noise is present during the conditions above, replace the evaporator assembly. If the noise is not present during these conditions, refer to workshop manual diagnostics.
WARRANTY STATUS: Eligible Under Provisions Of New Vehicle Limited Warranty Coverage
IMPORTANT : Warranty coverage limits/policies are not altered by a TSB. Warranty coverage limits are determined by the identified causal part.