I think the creaking / cracking is a small possible flaw with the unibody design (or, more accurately, how thin these macs are and inevitable small manufacturing differences and tolerances).
I've had it with a Macbook Air, old Pro and now a new Retina one. The retina occasionally has a crack noise if I grab it on the left side of the base, and ticks if I rest it on the middle of the base. I'm sure the crack is because of a slightly misaligned or loose screw on that side (so I'll give it a day or so and then re-torque it) and the tick in the middle is from the bottom case slightly touching the innards (I remember on my old MBP there were pins on the inside frame which made the base tick if you put the right pressure on parts of the base).
Of course this shouldn't happen, very expensive computer etc...but it does't actually bother me insofar as if it was bad I could get it sorted under warranty and when you think about it it's pretty inevitable given the material used. Aluminium will bend (with not too much pressure) and the base is under a lot of heat stress too and is close to the internals as tolerable.
I'd give it a day or so, let it heat up and cool down a few times then see how it is (that's my plan anyway!).
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P.s. I forgot to say that if anyone wanted to return it for this cosmetic defect they should. It shouldn't really happen. Me? I've realised seeking perfect from a made in China Mac is pretty difficult.