iPhone 4, 4S and 5 rattle inside?

Just wondering if anyone else has noticed a rattle from within the phone when you lightly tap the back toward the top of the phone near the apple logo. I've only noticed this with my iPhone 4 and my iPhone 5 and my friends iPhone 4S's, not from the 3GS or earlier. Anyone know what is causing the rattling sound? I'm just being curious, I know it's nothing to worry about. Thanks.

- ImageX

It might be the tilt switch (accelerometer) for the portrait/lansdcape screen orientation.

EDIT Apparently not though..check this out!http://www.engadget.com/2012/05/22/the-engineer-guy-shows-how-a-smartphone-accelerometer-works/

Upon review of a google search, it's the auto focus for the camera and most 3GS's and all iPhone 4, 4S's and 5's do this. As I thought, it's completely normal and okay. I just thought I'd pick some people's brains here before scouring google.

- ImageX