[gtg465x]

I ordered mid-October (is that late?) and the back on mine seems solid as well. Perhaps just a bad early batch?

No, I just ordered and received mine a couple weeks ago on November 18. At first mine was completely solid, but now the back definitely creaks even when holding it lightly. It doesn't bother me a ton, but it does make the phone feel cheap. I returned a Nexus 6P because I didn't like the screen (I think LCDs are superior and prefer them) and man do I wish the Nexus 5X was built the same way. The 6P feels super nice and high quality. Still, I enjoy the 5X more even though it feels cheap, is a little slower, and gets worse battery life. The screen is the part of the phone that you're always looking at, so if I'm not happy with the screen it kind of ruins the entire phone for me, even if the rest of the 6P is insanely good.

[gtg465x]

Its by design, unlike the Nexus 5, the 5X doesn't have adhesive strips to hold the back plate to the frame, just clips. A lot of people complained about those strips with the old N5 as it makes repairs difficult, so I'm guessing that's why LG removed them. This makes repairing the phone and removing the back plate much easier. It's also the reason why the back of the phone creaks when pressed.
I noticed this with my Nexus 5X and did some digging, a user explains it well in the nexus product forums,(i would post the link, but being a new user i can't ). This is also shown in the ifixit treardown of the Nexus 5X.

Interesting. Guess I'm wrong about it sounding like adhesive coming unstuck then.

[Zendroid1]

Mine does this too now. Just creaks when I press around anywhere on the back. Not very hard pressure either. It's a bit annoying.