2004 mercury mountaineer.heat problem When turning ac knob to heat clicking sound like door opening and closing repeatedly sounds and air blows cold instead of hot.

It sounds like the motor that opens the heat blend door is malfunctioning... They sell them for most makes and models at your local part store... Also a Haynes or Chilton manual to give you specific direction for your vehicle. Also try removing both heater hoses from your heater core at the firewall and now compressed air through the heater core... It may be restricted

This could be due to a failed blendoor actuator.

May be a restriction in the air intake of the HVAC unit or a door/flap closing for some reason not desired .

Your ac blend door is broken or the actuator that opens and closes the door is broken. The ac blend door moves up and down to direct air flow through the heater core(for heating) or evaporator core(for cooling).

When the door mechanism breaks, the blend door falls down and you will only have ac but no heating.

You can buy a ac blend door kit for your explorer, they cost about $120 on ebay or other places.
search for Explorer ac blend door kit.

The duct is under the the dash on the drivers side. And you will have to cut open the duct to install the replacement door.

Heres a link to a tutorial of how to do the job.

http://www.heatertreater.net/Ford%20Explorer%2002-08.html

there is a vacuum ran valve on your heater hoses goin in to your heater core on the fire wall, they tend to get stuck shut n dont open to let hot anti freeze in the heater core, so cold air right, qwik fix is find the valve on heater core inlet and wire tie it open so antifreeze circulates no matter what, this is known on ford explorers. pretty much same truck.

2004 mercury mountaineer.heat problem
When turning ac knob to heat clicking sound like door opening and closing repeatedly sounds and air blows cold instead of hot.

If I understand your problem, what you actually hear is a "clicking" sound. If I am correct, your unit only blows out cold air. If so, your problem is the "Heating & AC blend door actuator. It is a small plastic box that contains plastic gears. The unit receives an electric signal to change positions, but the broken cogs prevent the unit from operating properly. A quick check to determine if this is your problem, turn the heater on. Adjust the temperature from heat to air. At some point the noise will stop.

i had the same problem with mine only the noise was coming from the rear driver side where the rear ac motor is. the culprit was the motor that opens the baffle regulating the hot and cold air. the teeth on the the motor are plastic and a couple wore off and while trying to open the baffle would make the clicking sound. its a little hairy getting that motor out do to close quarters but a friend helped me and its fixed now. the motor cost me about 75 bucks. good luck hope this helps

Sounds like the compressor has failed.

The clicking noise is a defective air temp control blend door motor, this is all to common and issue, to repair the defective part the dash must be removed.