How to remove turn-style tub overflow cover?
The reason you hear a grating sound and can only turn the over flow plate slightly is because it is attached to a bushing which is screwed into the over flow pipe. When you turn the cap it threads into the pipe. If you are concerned that water is leaking from the over flow drain pipe behind the tub there should be an access panel on the other side of the drain wall (usually). If there is no way to check the drain you may want to create an access so you can check. Other possible ways to check would be the floor under the tub (if there is space).