I noticed my G3 creaked a lot when gripping it a certain way. I figured a little creaking is normal (coming from unibody HTC One m7). Checked my girlfriends g3 and it had no creaking whatsoever. Based on information from a different forum, i placed a folded up piece of paper between the battery and the battery door. This literally eliminated all creaking. I was concerned with the safety of this method so i removed it. After this i was playing with the battery cover and was able to bend it in ( so it's tight with the battery) and have virtually made it creak free!
Hope this helps anybody else whos OCD was nagging at them
UPDATE: Basically its caused by the battery being loose. Some batteries fit tighter than others. This can explain why some people have no creaking at all and others do.