Spoked with the Swedish Apple Care today and same reply, they don't know what it is. But I can return it if I want or replaced it. BUT, I talked again today with another guy from one of our biggest operators in his store, he showed me his (exactly the same as mine), the demo phone AND the 5C-demo phone. They all rattled.
He also told me that one customer replaced his 5S four (4!) freakin' times and it STILL rattled lol ^^
Why Apple don't know what the hell it is, IS a bit worrying. But the Apple-guy admitted too that this was very weird and that he didn't even heard about it. Like ever. But that's because I asked him if a 1200 EUR phone is supposed to feel "premium" like this. But what if I send this one for repartition, and the next 10? Haha, this is like a virus.
I will talk to another guy/victim and ask him a couple of questions. But no one so far has admitted if this is maybe manufactured failure. And IF it is and only 1% of the owners are not affected, that would be like everyone else wants new phones. Or in worst case scenario, get rid off 5S and buy an Android phone. ^^