I had some other issues as well and I was within the initial exchange window (ordered online so could return them without much discussion).
I've read about plenty of others with the creaking / cracking case sound either having the base plate tightened (with varying degrees of long-term success) or having the base plate / casing changed.
I did try tightening one of my cases and it reduced the cracking sound but never eradicated it unfortunately - I personally feel it's a slight imperfection in the case manufacture on certain units as it's (I think) all created from a single piece of alu.
Others just live with the sound too