My power button also rattles, it did on my old 5S and does on the new one. I'm not really bothered about it as i don't do much video but it does make the phone overall feel a little cheap. I'm now on my 4th iphone in the space of 3 months due to different issues. I am surprised as i've never had to swap a phone in the past, i used my galaxy S2 for 2 years and it was perfect.
I'm happy with my new handset even with a rattling power button but i am a bit shocked at the quality of iphones in general.
I've never had an iphone before now but i was expecting top notch quality but it just doesn't seem to be the case. I've had colleagues who have had to get their phones swapped in the past due to various issues, i've got no complaints regarding apple's service as it has been fantastic but my experience so far with iphones has been a bit disappointing which is a shame.