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Noise not only lures zombies in the area to the source of the noise, but if enough noise is made, new zombies will enter the area. The noise system is a big factor in the balance between melee and ranged, that is, while guns are powerful over a range, they also tend to be much, much louder than melee weapons. However, noise not only applies to the player's weapons/actions, but other humans as well. That means if a human enemy is making the most noise, the zombies will be attracted to them instead. Using certain items, the player could even lure zombies to their enemies.
Noise fades every round of combat, and so if the source of the noise stops, the area noise diminishes over time. Additionally, zombies (and human AI) are drawn to the site that the noise was made, not the person that made it. That means as long as the player can move out of visual range, their enemies may not even spot them. Eventually, if no new noise is created, the zombies (and possibly the human enemies) will wander off.
Noise ranges from 0-200 Decibels. At 0-25 Db it is indicated by a green number and thus only nearby enemies are alerted. At 50-99 Db (yellow warning) hostiles on the map are lured to the sound and finally at 100-200 db zombies from nearby areas are alerted and zombies will constantly spawn until the sound dies down.
Noise can also be used as a tactic. The player could set themselves up in a good position and use noise to lure zombies to them, or make noise to draw them out of buildings. Just remember that, if the area gets really loud for several rounds, the amount of zombies lured to the area can get dangerous real fast.
Area noise is derived from every single thing making noise in the area.
If you make a lot of noise, about 2-3 zombies will show up at the map entrances per turn assuming you are making the greatest amount of noise possible. If you reach the limit on the map, those zombies are queued up to spawn in as soon as other zombies are killed.
Noise is a local factor. If you leave the Combat Map, it's no longer an issue.