There is a knocking noise when I turn on the knob for the vent/A/C, why?

Hello, sorry for the delayed reply. I am out of town on business and just now logging on to catch up. The most likely issue is a failed component that directs air flow to the proper vent locations. The actual cause can only be determined by an evaluation by a competent technician. Please call us at (928) 783-8808 to schedule a convenient time to evaluate this issue for you. I will be traveling tomorrow and unable to respond further. Thank you so much for your inquiry and the opportunity to be of service to you!