I noticed the iPhone 4 (not sure about the 3GS) rattles when you knock or tap on the back part of the phone (near to the camera). Try to hold the phone on the bottom near the home button area in one hand, and tap the the phone against your knuckle or finger on your other hand in the camera area. It rattles.

Anyone else noticed that?

hmm yea, actually i've been using for quite some time but I start to noticed that after reading some post on the internet, i don't think its a problem, just that the sound makes us uncomfortable knowing the fact that the camera part is loose (due to something, which I'm not too sure what it is)...