2008 Mustang "clacking" noise
Sometimes, when we turn on the A/C on my daughter's, Mustang, a loud and steady "clacking" noise can be heard coming from the upper dash. Then, it goes away. Hard to explain, but, it sounds like something is hitting something. At times, there's also a hissing sound from outside the car.
Doesn't seem related to engine RPMs, and, the clacking noise is louder on the inside than under the hood.
Wish I could be more specific, but, I can't identify exactly how and when it happens, but, it is A/C or defroster related.