There are 2 types of sounds associated with AC. One is a whoop sound, which is related to the thermal expansion valve. It is loud and can be heard from outside the car. The other is a creak/squeak sound, which is the air vent direction changing. You can only hear this from inside the car. Someone standing outside won't be able to hear it. You can test the second by setting the AC to blow out of the face vents manually. If you then never hear the sound again while driving around, then it is the vent. Just live with it. If you still get the sound, then it is the whoop sound from the thermal expansion valve, which you might want to replace if it gets irritating.