I'm new to this car, 2010 mustang gt, and when I start it up there is a clicking behind the dash like a loud clock ticking off the seconds for about 15 seconds. What is it? Is it normal?

 

My 2005 does this too when I turn on the ac. I suspect a broken plastic arm connected to the actuator motor is my problem. Possibly yours too.

I think your car closes the vents when you take the key out and reopens them on start up.

I know I'm not much help but hopefully this gives you ideas in your quest. Watch a you tube video on how to open your dash and inspect/replace the actuator motor.

 

More help/info here:
https://themustangsource.com/forums/...ection-490082/