It is NOT the sleep/wake button. The Sleep/Wake button issue on the iPhone 4 was confirmed because when you rattled the iPhone 4 with the button held down, the rattle wouldn't be there. Now on the 4S, whether or not I hold the button down, the rattle is still there. SO ANNOYING!!!
Source: It happened to me when I had my iPhone 4 and I returned it to the the Apple Store for one that did not have a rattling in the button.
Source: It happened to me when I had my iPhone 4 and I returned it to the the Apple Store for one that did not have a rattling in the button.