It could either be the power button or the camera lens rattle.
My 4S has a slight rattle (from the camera), and you can only hear it at rare times when it's at a weird angle and you tap it with your finger, but my lock/power button is solid.
The 5 had this issue as well, especially had an issue with power button feeling loose.
The camera rattle isn't considered a defect by Apple, but a loose power button would be.
No it's not defective ffs. Enjoy your phone and move on with life. The power button needs some give so it may make some noise if shaken but most likely any sound is the camera lens.
It is the camera. My first iPhone 4 had a 'perfect' lock button. That had camera rattle. The second one had a 'not so perfect' lock button. For whatever reason there was no rattle. It's pretty normal with iPhones to have rattle and doesn't really affect things much.
It is a feature. Innovation. Competitors haven't even thought about implementing it. I just love it, I could not imagine an iPhone without that rattling.