Pontiac g6

Turning the AC on will typically run your fan at a different speed and alter the air flow dampers in the ducting system. I would start by determining if it is the fan since you should be able to turn it on and off at different speed settings without the AC on and listen for the noise. If is is not the fan motor, then you may need to expose the ducting system that runs through the dash if you are want to tackle it yourself. Have you ever stood outside the car with the hood open and listened for the noise while you have someone turn the AC on and off? It may take you in a different direction. Good luck.

My Pontiac G6 does the exact same thing. The A/C has to be on the internal circulation and not the outside function or else it will knock very loudly. I am kind of just dealing with it...