I don't think it's supposed to click. I think that clicking is a damaged flapper mechanism. The motor is trying to drive it, but instead the gear is slipping.So what happens is when i switch it to defrost mode, you can hear the blower die down, some clicking (actuator most likely), then it will start blowing again. There no air at this point anywhere! Then it stops again, does the clicking again, and then starts blowing out of the front.
What you can try is pull the air cabin filter. Apparently a lot of debris gets on top of it. Perhaps enough to cause problems.