2008 Ford Taurus

I have a 2008 Ford Taurus. Yesterday something weird started happening. When I have my air conditioner on everything is fine. When I turn off the air conditioner a loud knocking noise seems to be coming from the glove compartment/air bag area. The knocking is approx. (3) times per second. When I turn the air conditioner off, the knocking stops. Now when I get in the car and turn the key without starting the car, the knocking starts until I turn the air conditioner on. Can you tell me what’s going on here?