The door that controls the air duct is an electronic motor. That could be bad. My first thought is: Has anyone replaced or removed the interior air filter? I've seen some of these come apart and cause problems. Many times if you take your vehicle to Jiffy Lube or similar quick oil change shop they remove your interior air filter to wave in your face to let you see how dirty it is. If you refuse, they just cram the filter back into its place to be vindictive for you not buying a new one. Yes, I know this because I'm on retainer at some quick oil change shops.
Blend door actuator plays a role in controlling the air flow and temperature inside your TLX. Anytime there is a problem with the temperature of the air conditioning system, it could be a problem with the blend door actuator.
The most common symptom of faulty blend door actuator on Acura TLX is a slight clicking sound repeatedly coming from under the dashboard. The sound will be most prominent for a few seconds when you turn on the air conditioning or adjust the temperature. Sometimes changing the temperature can temporarily stop the sound.
A knocking noise could be another indicator of a bad blend door actuator on your TLX, although its quite rare. The sound is something like a light tapping on the door and it happens when you turn on the air conditioning system or turn off the engine.
Does anyone know of the HVAC self diagnosis steps on the TLX? It identifies the problem area in the HVAC system. On previous previous Honda's, you press and hold a couple of buttons while it gives you a code. Usually one of three like mode control, temp control or blower motor.