1 - Pretty sure the noise originates at the player, so that would be a bug.
2 - Your sneak meter is based on your light level, not what any specific zombie can or can't see. It the zombie facing you? AI does not see light on walls. It sees you or not.
3 - When you get near a volume it spawns, so you may see things appear. Just like a lot of other stuff appears out of nothing in games. Low FPS really breaks 'immersion'.
4 - Yep