Friend of mine just recently got a Bersa Thunder .380. He field stripped, cleaned and oiled it. We both noticed that when racking the slide, there is a peculiar sound. It kinda has that tone of metal rubbing against plastic. If there was a word to describe it, it would probably be zwip. It may be nothing, but he does not want to fire it until he knows if this is normal. Otherwise, me thinks that it is back to the factory. Is this sound normal?