I don't know why this happens, but no matter where I am, no matter what map, and what server, I will occasionally hear one of these 2 sounds, either what sounds like a metal gate swinging open or shut, or what sounds like someone trying to open a locked door (a door that's been lockpicked). I've gone to the middle of the ocean, been in the middle of a field, even outside of the map and I still hear it, it's very strange.