As someone who's returned an iPhone 4s and an iPhone 5 for this, I can assue you that the majority of them do this -- all to varying degrees.
My first 4s sounded like a sack of nickles when I'd tap it or put it down. Got a replacement that was MUCH quieter but still had it. My wife has the phone now and it doesn't bother her. It's pretty prounounced, especially if you listen for it.
My iPhone 5 had the same "issue" upon arrival. Could it have anything to do with the p*ss poor packaging that they do when shipping via fedex...? Nah...
Not horribly discouraged, I pondered it and rolled the dice for a new one hoping to have the same experience with my 4s (not eliminated but drastically reduced).
Was it reduced? a fraction of an amount. If the "intensity" of the rattle was a 9/10 it went to an 8.5/10 ha ha. Totally not worth the headache of replacing (long story).
I'm with cfcboy and would almost wonder if it's abnormal to *not* make any noise... I've done a lot of reading and posted many links in other threads to camera lens design snippits found on the internet that pretty much confirm that, if no "locking" mechanism is in place to lock the moving AF lens in place, it will wiggle around in there. If it doesn't make any noise, perhaps it's out of the tolerance and stuck or there's a problem with the AF lens.
Either way, my advice to others is to do your best to ignore it. Every single iPhone 4s and 5 that I've ever touched has done this. You won't see it as much/bad with the 4 as it was different. You're likely to see it with every 4s and 5 to some degree.
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...also for those of you wondering how to test this you'll need to eliminate the sleep/wake button by touching it with one finger. This wiggles ever-so-slightly on both of my 5's (didn't on my 4s) so I think it does more on the 5. Anyway, open the camera app and start tapping the back to repro the nose. As you tap, tap the screen with the other hand to activate the AF lens. Note that during the 1-2 seconds that the AF mechanism is working, the rattling nose gets drastically reduced (since the lens is being actively used).
Try it!