Is it the aluminium itself or the case you've put around it?
Regardless there have been issues with bottom case flex, especially on the classic unibody shell (2012 or earlier). Not as much of an issue with the Retina models as they're built to much higher tolerances, and the 2016 even more than that.
It can't hurt to take it to an Apple Store. If it is the bottom case flexing, then unscrewing it and reseating it normally does the trick. They'll have it sorted for you in a jiffy.