With pickup at a stop, blower set on max, vent set on full defrost, and temperature setting on cool, a loud whistle comes from the vents on passenger side.

Which of these two systems does your truck have?

Electronic Automatic Temperature Control (EATC) System
Or
Electronic Manual Temperature Control (EMTC) System


Electronic Manual Temperature Control (EMTC) System
The electronic manual temperature control (EMTC) system heats or cools the vehicle depending on the EMTC control panel selection.

• The EMTC control panel selections determine heating or cooling, air distribution and enables blower motor operation.
• The temperature control setting determines the air temperature.
• The blower motor switch varies the blower motor speed.
• During A/C operation the system also reduces the relative humidity of the air.

The EMTC system operation is determined by the settings on the climate control module. The EMTC system is composed of the EMTC module, blower motor resistor, mode door actuator, air inlet door actuator and temperature blend door actuator to control the various functions.
The blower motor switch:

• sets the blower motor speed.
• directs the blower motor path to ground through the blower motor resistor to allow blower motor operation in LO, MED LO and MED HI.
• directs the blower motor path directly to ground, bypassing the blower motor resistor, to allow blower motor operation in HI.

The A/C request button:

• can command the A/C compressor ON when the function selector switch is in PANEL, FLOOR/PANEL, FLOOR/DEFROST and FLOOR when the A/C request switch is pressed. Indicator illuminates when A/C request switch is toggled ON.
• is inoperative in MAX A/C, OFF and DEFROST.
• indicator illuminates when the function selector switch is in MAX A/C and cannot be toggled.
• indicator does not illuminate in OFF and cannot be toggled.
• indicator does not illuminate in DEFROST but can be toggled ON or OFF. The A/C compressor will operate regardless of indicator status if the outside air temperature is above 6°C (43°F).
• is serviced only with the EMTC module.

The function selector:

• selects airflow direction.
• is serviced only with the EMTC module.

The temperature selector:

• selects full warm and full cool as well as blended outlet temperatures.

The air recirculation button:

• selects either recirculated or outside air source.
• recirculates air in any mode except DEFROST, if requested.
• is disabled in MAX A/C or OFF mode where recirculated air only is used.
• is disabled in DEFROST mode where outside air only is used.
• is serviced only with the EMTC module.

The EMTC module also includes:

• a heated mirrior button. Refer to Section 501-09 .
• heated seat controls. Refer to Section 501-10 .

As per your problem mentioned. There are few solved help links to troubleshoot these problems. Go through the list and Click the link below:----

2000 Toyota Avalon: No heat on Passenger Side?

http://whatisbyme.blogspot.in/2012/05/2000-toyota-avalon-no-heat-on-passenger.html

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No heat on Oldsmobile Silhouette?

http://technoanswers.blogspot.in/2012/02/no-heat-on-oldsmobile-silhouette.html

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Heater blows cold?

http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.html

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Heat blows cold on drivers side and hot on passenger side?

http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html

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Hot air on the driver's side vents only?

http://repairhelpcenter.blogspot.com/2012/01/hot-air-on-drivers-side-vents-only.html

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Temperature gauge does not move and no heat when heater is ON?

http://technoanswers.blogspot.com/2012/01/temperature-gauge-does-not-move-and-no.html

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1992 Toyota Camry: A/C Control light Flashing and A/C not Blowing Cold?

http://technoanswers.blogspot.in/2012/04/1992-toyota-camry-ac-control-light.html

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No heat, blower blows only cold?

http://technoanswers.blogspot.in/2012/02/no-heatblower-blows-only-cold.html
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Ford F150 Ac only blows through defrost vents?

http://technoanswers.blogspot.com/2012/01/forf-f150-ac-only-blows-through-defrost.html

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Dodge A/C come out of defrost vents only?

http://technoanswers.blogspot.com/2012/01/dodge-ac-come-out-of-defrost-vents-only.html

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Ac vacuum line on dodge cummins?

http://technoanswers.blogspot.com/2012/01/ac-vacuum-line-on-dodge-cummins.html

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Ford ranger AC blows cold through defrost vents instead on cabin vents?

http://technoanswers.blogspot.com/2012/01/ford-ranger-ac-blows-cold-through.html

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2005 Chrysler PT Cruiser: A/C Blows Warm?

http://whoisbyme.blogspot.in/2012/06/2005-chrysler-p8t-cruiser-ac-blows-warm.html

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2009 Mercury Grand Marquis: A/C blows Cold From Defrost Vent Only?

http://whoisbyme.blogspot.in/2012/06/2009-mercury-grand-marquis-ac-blows.html

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AC not blowing through Vents on 2007 Chevy Trailblazer?

http://schematicsdiagram.blogspot.in/2012/04/ac-not-blowing-through-vents-on-2007.html

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Passenger side temperature is warm air out of dash vents?

http://schematicsdiagram.blogspot.in/2012/04/passenger-side-temperature-is-warm-air.html

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These will help.

Thanks.

Got kids? Years ago, mine loaded the defroster ducts with toys, candy, etc.
Otherwise, please create another post and identify the year, make and model vehicle you're talking about before posting your question.

AC Actuator ?
The temperature control regulates the HVAC system air temperature in any mode. A temperature control cable mechanically links the temperature control to the temperature door. Your hearing stuff inside the blower motor housing , like leafs or someother de·bris .

Because your vehicle has no electric actuator .
Disconnect the electrical connections from the blower motor.
Remove the blower motor retaining heat stakes with a chisel or a screwdriver.

Sounds like the blower motor FAN cage came lose or broken? could be bad blower motor itself ???

SECTION 412-02: Heating and Ventilation 2000 Focus Workshop Manual REMOVAL AND INSTALLATION Procedure revision date: 10/6/2003 Blower Motor Removal
All vehicles

1. Remove the footwell lower trim panel.

1. Open the glove compartment.

1. Remove the footwell vent air duct.
   • Pull off the hose.

1. Close the glove compartment.

Vehicles with electronic automatic temperature control (EATC) built 11/2003 onwards

1. Disconnect the blower motor control module electrical connector.

All vehicles

1. Disconnect the blower motor electrical connector.

1. Remove the blower motor.

Installation
NOTE: Procedure if there is a concern over whistling noises from the heating system.

1. NOTE: Set the air distribution switch to ""Head area” and the temperature control switch to ""MAX cooling”.
   Remove the ventilation center nozzle and open the glove compartment.

1. Unclip the passenger side defrost nozzle air duct and position it on the open glove compartment cover.

1. NOTE: Do not allow any parts of the joining tang to pass behind the instrument panel.
   Remove the two-part plastic joining tang on the heater housing.
   • Cut the joining tang at the top end using side cutters, break it off at the bottom and discard it.

1. Cut the sealing tape into a 50x50 mm square.

1. WARNING: Sharp edges on the heater housing.
   Feel for the gap in the heater housing, affix the sealing tape over the gap from the inside and press it firmly in place.
   1. NOTE: Apply the sealing tape in the position shown. The sealing tape must not be applied higher than the position shown in the illustration.
      Heater housing (shown in cross section)
   1. Sealing tape

1. To install, reverse the removal procedure.

When the engine is cold,check the coolant level.

Make sure it's on max.

===

A coolant system should be flush every two years.

===

If you did a coolant flush recently then air pocket can be the cause of the un-even heating problem.

The DIY method is to drive you car onto a ramp or a steep hill.

Iet the engine cool off and top off the coolant tank.

Set MAX heat + MAX fan

Let the engine ran for 20 minutes wih the cap off and the trap air will escape from the coolant reserve tank,

Check coolant level and top off coolant when the engine cools down.



A shop will charge $75+ for this but using an air operated excavate tool!

========

A car with over 90K miles may experience linkage problem to the heater control valve.

The heater control valve (near the firewall) may be stick over time.

Open the hood and move the heater to max and min. Back and forth.

Clean and lube the linkage with WD-40 to free the heater control valve.

If it's a vacuum type heater control valve then cash for leaky vac. hoses.


=======
Move the fan switch back and forth to re-new the electrical conductors.

Pass a vacuum over the control panel to prevent dust build up behind the console.

Before replacing anything, look for a vacuum leak. Check under the hood for a broken or dislodged vacuum hose.

There is a vacuum leak under the hood. Find the vacuum line on the passenger side where it comes through the firewall and follow it to the engine. You system needs vacuum to operate and without it the default setting is on defrost for safety reasons.

Concerning your broken vent -- this can be caused by a broken linkage or a malfunctioning servo motor as well. these vent have air ducts the sway back and forth to adjust the dual air flow function. if the servo o linkages are malfunctioning or not connected for some reason, it will not allow total control of the air flow and temperature range. Now, if you have a combination of a flow error and temperature error as well, thai can be caused by a combination of the two.

In this case, you will need to inspect the linkages as well as the servo motor function as well. in addition, i would test the air temp sensors for there operational value. they control the temp when you adjust the coll or hot function.

Concerning the Loud noise coming from the blower fan---

This issue can be caused by a misalignment within the blower housing or, it can also be caused by some minor damage within the motor itself, causing it to rotate in a erratic manner, in return, forcing it to rub against the housing itself. the best solution here is to extract this blower fan housing and inspect for lose fan propeller and motor stability. replace parts accordingly.

PLease rate and have a great day:)