Hi and thanks for using 2CarPros.com.
Based on your description, I feel it may have to do with either a blend door actuator or a mode door actuator. These actuators are small electrical motors which open and close different vent doors in the vehicles HVAC system based on what you are requesting the system to do.
What I need you to do is pay attention if you hear the noise after changing something on the air conditioning or heating system, whether it be temperature change, air flow direction, or fresh to recirculated air changes.
Here is a link which describes what happens, how it works, and general directions for changing one of the actuators. The first paragraph indicates with the key on. However, it can happen when it is turned off, too.
https://www.2carpros.com/articles/replace-blend-door-motor
Once you do this, let me know what you find.
I hope this helps.
Take care,
Joe
Jul 12, 2018 at 8:35 PM
