I actually thought this was from using an InCase hard shell case, which kind of fit but really didn't, figured I loosened the hinge tearing off the shell, which was held on pretty well.
Turns out it was just the screws on the bottom cover of the Macbook Pro! I had recently taken the cover off to dust it out (no dust!), and kind of overtightened the screws, purposely because I wanted the screws flush with the shell, not sticking out in any way.
But again, they were in too tight, so as per CoolDuckie's post, I slightly loosened all the screws and violá! The creaking is gone! I'm not really 100% sure why that overtightening of the backplate caused creaking in the hinge, but hey, it's fixed and I'm happy! It's also noticably easier to open the Macbook Pro when it's closed.
It'd be cool to know exactly why this solves the problem, anyone know?