This is a common problem on the Explorer. The actuator motor that is clicking is on top of plenum box behind the glove box. It is difficult to remove, but not impossible. The usual cause of the problem is a broken blend door that has allowed the motor to get out of alignment and the clicking you hear is gears internal to the motor casing slipping.
You can pull the motor and replace it, but that won't usually solve the problem. You have to go after the blend door. There is an aftermarket repair from HeaterTreater that will avoid the dealer cost of removing the dash and plenum box to replace the plastic door. Check heatertreater.net for diagnostic information and a cheap easy fix.