There is an electronic part that controls the air condition outside and inside circulation. It is behind the Glove Compartment. To change it, remove the Glove compartment, and you will see it on your right hand side. Just a few screws and an electrical plug and that is.
As soon as you change it, the knocks will end. That part is known as an actuator with the following numbers:
Saturday, September 11th, 2010 AT 4:22 PM