Yep, my old iPhone 4 had the same problem just much worse. I can hear it really well when i put my phone down on the table etc, and when i get a text while im on the phone the vibrate makes it rattle. I think any moving part is bound to rattle!
Looking at the poll, I'm thinking if I do swap my phone tmr, there is a good chance i will end up with a phone that has a rattling power button and other cosmetic flaws. Can't help but compare my odds of getting a perfect phone with the roulette table