Leave the A/C at anything but 75°F either full cold or full hot..
If It do the sound = Then nope its not the Air Blend Motor.
If it doesn't do the sound = It is the Air Blend Motor.
The problem with the Air Blend Motor relies on it trying to compensate for different temps at 75°F setting it will try to either blow hot or cold and since the threshold is at 75°F it will keep trying to switch to either setting. When you leave the A/C off at either full cold or full hot the Motor wont try to switch since it will need a massive temp difference to make it cycle.