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My experience/opinion:

I've a 16GB & 32GB 4S. Neither rattled on vibtrate or when shook gently (yes, if you tap near the camera, there's a rattle, imo to be expected).

However, the 16GB phone was replaced after I received it with scratches along the bezel (not scratches from contact scrapes, but looked more like marks from the machining/manufacturing process).

My replacement 16GB phones rattle when it vibrates/shook gently; it's not the mute/volume/power switch, but rather appears to come from within the phone. imo not normal, as not all 4S's are like this (based on a sample size of 3, 2 didn't rattle).

So 2nd phone is going for replacement (third time lucky, maybe). Perhaps wouldn't bother some people, but I'm not willing to accept manufacturing defects on a phone costing £500.
 
I can indeed verify its the camera focus, try this: open camera, focus on something as close as you can, try tapping and the rattle goes away! Focus on something far away, and the rattle comes back.
 
I have the same issue with the rattling, but I realized that if I hold down the camera lense area, and then tap on it, the rattling doesn't happen. It only happens when putting the phone down on a hard surface. After readin this thread though, I don't think I'll worry about it unless it starts to change how the phone works. I have a friend who fixes iphones and get tighten it for me if needed for a lower cost then getting a replacement, as verizon wants $169 to replace even with the insurance and warrranty..which is rediculous.
 
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