I had to return my first unit due ear speaker defect and it also had squeaking back cover. When i got my new unit and after putting sim and memory card in i noticed that back cover doesn't squeak at all. I then realised what was different.
The way i opened my first unit cover was that i lifted it from bottom first all the way open and only then long sides. Obviously that bended cover and it started squeak when it didn't fit properly any more.
The way i opened my second unit cover, was that after lifting bottom corner open i then proceeded to long side next and then top short side, when finally whole cover popped off. I installed it back in reverse order and it was very snug and no squeaking at all.
I can't be 100% sure if that is reason for squeaking covers, but at least that can be one explanation.
The way i opened my first unit cover was that i lifted it from bottom first all the way open and only then long sides. Obviously that bended cover and it started squeak when it didn't fit properly any more.
The way i opened my second unit cover, was that after lifting bottom corner open i then proceeded to long side next and then top short side, when finally whole cover popped off. I installed it back in reverse order and it was very snug and no squeaking at all.
I can't be 100% sure if that is reason for squeaking covers, but at least that can be one explanation.