I just started developing this odd creaking noise near the bottom and sides of my S4G. Even with a Seidio Active case wrapped around it, the creaking can be felt and heard when hitting the bottom buttons.
The fix is pretty easy. You just need to decrease the amount of movement the back cover has.
I had some .010 mil fiber (fish) paper laying around so I cut a small square that covers the sim card and microsd card area. You can probably use something else, but hey, why not use some insulated paper if you have it.
Lay the paper on top of the area and then slowly place the back cover back on. No holes are in that area above the sim and microsd, so no worries there. You should see an immediate improvement.