I just tried it, and sure enough there's a very light click/creak sound that doesn't happen all the time. I never would've noticed (and in fact didn't until you said something, despite having a STR since June). It seems like it's maybe a slight looseness of the bezel around that area but it doesn't seem like it's a problem or will get worse over time. Well, unless you can't stop squeezing it now that you know it's there

.
Looking at
teardown photos, I suspect that it's because of the cut-out in the frame required for the USB port. That part of the back doesn't seem to have as many clips as the top or sides.