2000 gmc 2500 pickup heater fan works good but not a lot of air

Usually a sign of a faulty blend door under the dash.
The blend doors direct air thru the heater core and the desired vents.

Usually a sign of a faulty blend door.
The blend doors direct air thru the heater core and out thru the vents.

Usually a faulty blend door under the dash. The blend doors direct air thru the heater core and the vents.

If the compressor is running and one of the ac lines is cold, you probably have a blend door stuck or faulty in the HVAC box under the dash. The blend doors direct air thru the heater core and evaporator core as well as thru the vents.

Under neath the venting system behind console there is a venting housing that can be removed. Likely the air flow directional vent flap is stuck or the motor that moves the vent is faulty.

There is most likely a cable that has come off or broken behind the heater controls

good work, the clicking noise you hear is the blend door trying to move. this door is what controls the temp. the most common reason is the actuator motor is bad but sometimes the door hinge breaks, this hinge slides into actuator and is used to move the door. you will need to locate it on the heater box by locating the noise and then remove the actuator and see if the hinge is broken, i will try to enclose some picture to help you. good day, hope this helps.

check linkage on the vacuum motors that run those mode doors?

It is a vent door actuator motor on top of the blower box,the dash has to be removed and set in the seats to replace the actuator motor.Cost for the part is about 50.00 dollars

sounds like bad acuator motor, this is what moves the door in heater box to set hot or cold air, and other motors to set air to blow thru defrost, vent or floor. good luck.