FYI the rattle is normal. Think it's the vibration mechanism. My 4s had it too.
I did some a/b testing withe the 5 and 4s before I sold it and I was hard pressed to hear a difference in sq. I used my vmoda m80 and ultrasone 580's. I did notice it seeming more clear(or digital as others said) but I could not tell the difference between the 2 for the most part. Not enough to return the 5.
No I think the rattle on mine has to do with the camera lens cover. 2 of my friends have 5's as well and there's doesn't seem to do it. Hmm. I also discovered that the Auto Brightness settings doesn't do anything on mine. Meaning there must be a fault with the light sensor or something. I'm glad I discovered this though. I was really upset at myself for dropping this thing barely a week after I had it lol.