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Noise attracts all zombies in a general proximity towards the player's presence. The level of hearing zombies possess can be changed in the sandbox settings before starting the game. Some actions generate more noise than others, and therefore are more dangerous to perform. There are many actions that make noise that will attract zombies. These include the following:

Noise can be muffled by walls, doors, and distance between the generator of the noise and zombies.

Sometimes the metagame will have helicopters fly overhead, or random gunshots fired to move hordes of zombies around the map.