Thanks for using Just Answer, the most common cause of the problem you are having is a defective temperature blend door actuator. That's the clicking sound you are hearing, I am sending you instructions in pdf format on how to replace, hope this helps you out.
The temperature blend door actuator:
Before replacing the Blend Door Actuator, make sure cooling system is full and there are no air pockets trapped in the system, also while the engine is running make sure both heater hoses are very hot, if one is cooler than the other suspect a plugged or restricted heater core.
If the hoses are only getting warm, (must be very hot) suspect a defective thermostat, as long as there is no air in the cooling system.
I am sending you the istructions in pdf format, on how to replace the blend door actuator with Manual Controls, hope this helps you out.In order to replace the Blend Door Actuator you will have to remove the instrument panel, (CLICK HERE FOR INSTRUCTIONS).CLICK HERE FOR FLOOR CONSOLE CLICK HERE FOR TRIM PANELSCLICK HERE FOR BLEND DOOR ACTUATOR INSTRUCTIONS2004 Explorer/Mountaineer Workshop Manual
WARNING: Always wear safety glasses when repairing an air bag supplemental restraint system (SRS) vehicle and when handling an air bag module. This will reduce the risk of injury in the event of an accidental deployment.
WARNING: Never probe the connectors on the air bag module. Doing so can result in air bag deployment, which can result in personal injury.
WARNING: Never probe the electrical connector on a safety canopy module. Doing so can result in safety canopy deployment.
WARNING: Do not handle, move or change the original horizontal mounting position of the restraints control module (RCM) while the RCM is connected and the ignition switch is ON. Failure to follow these instructions may result in the inadvertent deployment of the safety canopy and risk of personal injury.
WARNING: To reduce the risk of personal injury, do not use any memory saver devices.
NOTE: If a seat equipped with a seat mounted side air bag and/or a safety belt pretensioner (if equipped) system is being serviced, the supplemental restraint system (SRS) must be depowered.
NOTE: The air bag warning lamp illuminates when the RCM fuse is removed and the ignition switch is ON. This is normal operation and does not indicate a supplemental restraint system (SRS) fault.
NOTE: The SRS must be fully operational and free of faults before releasing the vehicle to the customer.
WARNING: To avoid accidental deployment and possible personal injury, the backup power supply must be depleted before repairing or replacing any front or side air bag supplemental restraint system (SRS) components and before servicing, replacing, adjusting or striking components near the front or side air bag sensors or RCM, such as doors, instrument panel, console, door latches, strikers, seats and hood latches. The side impact sensors (if equipped) are located at or near the base of the B-pillars and C-pillars. To deplete the backup power supply energy, disconnect the battery ground cable and wait at least one minute. Be sure to disconnect auxiliary batteries and power supplies (if equipped).
WARNING: The restraint system diagnostic tool is for restraint system service only. Remove from vehicle prior to road use. Failure to remove could result in injury and possible violation of vehicle safety standards.