My Shield 9mm does the same thing.
I found it while I was cleaning and messing with it when I first bought it.
I called S&W and they gave me the address of a location local to me where a S&W armorer worked.
I took it to him and he disassembled the gun and checked it out, he said it wouldn't hurt anything.
He fired a few rounds to check the dimple the firing pin made on the primer, it looked good, he handed me my gun and said it won't give me any trouble, shoot it and don't worry about it. So I have been shooting it and I haven't had a bit of trouble with it.