Need to replace A/C actuator where is it

This is probably the Air Blend Door Actuator. There is a small gear driven motor that opens and closes the various heating and air conditioning vent doors inside the dash. Since you hear it after turning the car off, it is most likely the fresh/recirculating air door, and they are designed to automatically open to allow fresh air in when the car is turned off. The clicking you hear is the sound of the gears slipping inside the actuator. The parts are fairly cheap ($20-$35) and with simple hand tools can be replaced.
Here is a video showing them replacing one on a Monte Carlo, which is basically the same as the Impala:

I have to replace the actuators on Tblazers and Evoys all the time, usually the fresh air or blend door actuators but sometimes it ends up being the mode actuator which on some models is a little harder. Find which button on the control panel effects the clicking,usually made easier after the noise has stopped.Find which control button starts it back clicking

Usually this is caused by a binding blend or mode door. The actuator keeps trying to move it and can't. Locate the exact source of the noise. Then disconnect the actuator and try opening and closing the door by hand.

The noise is one of the actuators that moves the air flow doors into different positions. Common problem. When the noise happens, move the temp. control from cold to warm. If the noise goes away, it is the temp. actuator. Or, press the recirculation button to go from fresh air to recirc. If the noise goes away, it is the recirc. actuator. Or, change the mode from vent to heat. Could be the mode actuator. Usually the temp actuator acts up.

I doubt if you have a fan problem. More than likely you have a bad actutor motor. There are 2 small electric actuator motors under the dash. The temperture door actuator moves the air temperature door, which controls the temperature of the air coming from either the a/c or heater vents. The mode actuator (which is probably the one you hear clicking) controls the mode of operation, or more simply, which vents the air will be coming out of. When the internal gears of the mode actuator fail and start slipping, they make a loud repetative "clicking" sound, which is what you are hearing. Another symptom that may or may not accompany this situation is a longer than usual wait time for your mode selection to occur. So to sum up, a loud repetative clicking sound from the center of the dashboard possibly accompanied by a longer than usual wait time for your ac,heat or defrost mode to occur, usually indicates a failing mode door actuator.

#04-01-38-002A: Clicking Noise From Instrument Panel After Vehicle is Started (Replace Mode Valve Actuator Assembly Using Special Tool GE-47676) - SUPERSEDED BY BULLETIN 05-01-38-001A - (Oct 21, 2005) Mode Valve Actuator Replacement Tools RequiredGE-47676 Mode Actuator Removal Tool
Removal Procedure

1. Remove the left floor duct. Refer to Floor Air Outlet Duct Replacement - Left Side .
Important: The service actuator is positioned in the bi-level mode. Place the mode door in the middle of the door's travel.

2. Disconnect the mode actuator electrical connector.
3. Remove the mode actuator mounting screws.
4. Using GE-47676 remove the mode actuator.

Installation Procedure

1. Install the mode actuator.
Notice: Refer to Fastener Notice in the Preface section.

2. Install the mode actuator mounting screws.
Tighten
Tighten the screws to 1.9 N·m (17 lb in).

3. Connect the mode actuator electrical connector.
4. Install the left floor duct. Refer to Floor Air Outlet Duct Replacement - Left Side .

sounds like the temperature blend door is either jammed or the actuator is shorted ou. when the temperasture control is moved you should hear the door moving...my guess is you are not hearing that..the actuator would be attached to the heater plenum behind dash/glovebox area...becareful to test the correct one as there will be a fresh air actuator and a mode door actuator as well...you can determine which one is which by changing the positions on heater head from hot/col, floor/defrost, recirculate and fresh air settings..each setting will move its designated actuatror..the one not moving would be you temp actuator....have fun!

Sounds like the air vent actuator has gone bad and will need to be replaced. Try changing the settings to your vent and temperature to see if the noise goes away or you can try resetting it but disconnecting the battery for 2 min and reconnecting it again.
The vent actuators will sometimes loose there knowledge or position and will try to keep moving the vent, even though it is at it's limits. If resetting it does not help, good chance is that the plastic gears in the actuators have gone bad and the whole unit will need to be replace if you have lost any controls to your vent or air temperature.
Thank you for using fixya and please keep us posted.

this may be a solenoid clicking behind the glove box remove the glove box and you will find the relay thats clicking

the problem is the vent door actuater you can get it at your local parts house i got it at orileys auto parts