Mine came loose and rattling right out of the box. The area around the camera also pops and clicks when pressed and that also seem to make the camera pop out and become loose.
Even when it is properly retracted, the hinge is loose on one side and the module can be moved vertically up and down. It definitely seems like a design flaw and makes me concerned about the longevity of the product.
I noticed that some of the reviewers noted the same thing, from Android Central about the camera module:
"I'm starting to notice a slight wobble with the sensor even when it's not engaged. The motor itself still engages smoothly, and I haven't run into any issues, but I am worried about long-term durability. There are a lot of moving parts in the module — ASUS says it used 13 gears to provide a smooth rotating motion — meaning plenty of chances for something to go wrong."
If your unit has a loose hinge on one side, please contact your local ASUS repair center for a hardware repair. It sounds like something is wrong with your unit.
I have the same issue, but the problem is always resolved by clicking the "retract camera" button within Android's "Quick Settings" menu for me.