That knocking noise is probably a blend door actuator that has gone out and needs to be replaced. Here is a guide that will help you see what I am taking about with a diagram (below) that will show you what it like on your car.
Repair Safety Notice: This information is for general instructional purposes only. Vehicle repair can be dangerous. Verify all information, follow manufacturer service procedures, use proper tools and safety equipment, and consult a qualified repair shop when needed.