Should my MacBook Pro be creaking? After keeping my MacBook Pro open for an hour or so, it will creak loudly when I close it. Is this normal? If not, what should I do about it? It's a late 2011 MacBook Pro 13".
This is not normal. I had a very similar problem with a unibody Aluminium MacBook (they only sold them for a short time in 2008. This is nearly the exact same case as the MBP that you have, except your Pro did actually have minor updates to help alleviate the problems we had in our MacBook. 

The problem would only appear after the machine had warmed up for a little bit. The hinge would make a creaking noise when closing, and as it got worse, when opening. It also would stay hinged just slightly open (not waking from sleep) when being carried in a laptop bag. 

This was due to an issue in the hinge on the keyboard part. Apple replaced two screens for us. The problem would fix it self for a month or two, but them come back. They finally redid the entire top half of the machine, and it worked great for 9 months. 

2 months out of warranty, it happened again. Due to having over 3 repair attempts (there were actually 4), Apple instead offered us a replacement machine. We were told at that time that there were quite a few reports of this problem, and they had been addressed in the latest rev of the MBP. Since the unibody MB was no longer available, they upgraded us to the base 13" MacBook Pro. Im assuming that while it mostly had been fixed through minor updates, there may be one with a similar problem now and then.

The problem is related to the hinge design, and I have only seen it affect 13" models. I would highly recommend taking it to the Apple store and having it replaced. They will not replace the whole top part on the first try, only the display.