Just upgraded to iPhone 5s yesterday after 2 years of using an iPhone 4S. One thing is bothering me -- when I open up games, it sounds like a fan turns on and is noisy. It's some sort of whirring noise that sounds like a fan to me. Who even knew that iPhones had fans? I don't recall ever noticing this in my 4S and yet I noticed it on my first day with my 5s. I noticed it while playing Dead Trigger in bed at night with the audio off. The fan noise goes off a few seconds after closing the app. It does this when I open Candy Crush as well. Is this normal or is my unit defective? I do have a Mophie juicer case on it if that might make any difference?