to locate the flap motor / servo you can usually look up under the dash towards the center of the vehicle and see the white plastic lever moving rapidly back and forth.
The purpose of the servo is to mix cool air from the A/C or outside with warm air from the heater
to match the temperature you have set. The white lever
has a travel of 90 degrees.
One extreme is all cold air (no hot air mix), and the other extreme is all hot air. The A/C computer sets the position of the servo based on the air temps and the desired temp setting. For the servo to make the noise, it has to be in the middle region between the two extremes. This is generally when the set temp is close to the actual temp. You can try moving your set temp from very cold to very hot and watch the white lever move from one extreme to the other.
Note that if it's too hot
or too cold, you might not be able to get the servo to stay in between the two extremes. When the lever is in between the two extremes of travel, it should be very stable although it may move a bit every minute or two
so I'd suggest replacing this servo as the next step