@ the OP...
Yup...One of the units I returned a few weeks ago which was actually a 2012 rMBP 13 inch variant had that issue. The sides also creaked like crazy as well.
I ended up returning that unit and finally got a good one with no creaking or moving parts. It looks like some of the build quality issues that were present on the 2012 units are still making their way into the early 2013 units.
If it bothers you take it back and exchange it for another one, these machines are not cheap and you should get what you paid for...Which is a well built machine that was put together correctly...but thats just my 2 cents.
Good luck