Titan XD Diesel Pro4X
I have the same ticking sound coming from the dash when I use the controls for the heat. I took it to the stealership and they told me it was the metal cooling down. I know it doesn't make any sense to me either. I have had it going on for quite a while now, I just ignore it, if you figure out what it is though please let me know I am curious about it.
The clicking sound is the plastic gear that rotates the flapper from one position to another, i.e. from the head and foot mode to just the head mode. There is a write up on here as to how to change out the gear if you find it just too annoying to live with. I read through the write up and it seems extremely simple, the writer of the procedure said the hardest part was just trying to get underneath the dash to see what you are doing when trying to change it out. Mine does the same thing, but it doesn't seem to bother me. I will live with it until the gear finally strips out and quits working, then I will attempt to get my big butt under there and change it out.