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If you aren't having trouble with changing temperature or mode then most likely you are hearing the recirculation door actuator. The recirc door is the door that opens and closes to allow the heater box to draw air from inside or outside the car.
The problem could be the actuator itself, the door could be broken or it could be physically stuck.
The recirc door can be accessed through the glovebox opening. The first thing I would recommend is dropping the glovebox down and inspect the door to see if it's broken. If not then you can remove the actuator and move the door by hand to see if it is stuck or possibly isn't able to go to full travel in each direction.
thank you - I'll check that out first