Zombies hunt by sound first. Every loud noise in a dead city brings more of them. One zombie is a chore. Twenty is a death sentence. Staying quiet is the most important rule in this world.
Quiet. Melee, bows, crossbows, a suppressed gun, whispering, walking carefully. In a dead city, nothing extra comes.
Loud. A gunshot, shouting, breaking glass, an engine, a barking dog, a Molotov. In a dead city, a Zombie Pack arrives in 1d4 rounds.
Alarm. An explosion, a car alarm, a siren, many gunshots in a row. In a dead city, a Zombie Horde arrives in 1d4 rounds, and another every minute after.
Dead City. The full rule above.
Wild Land. Half as likely. A simple way to run it is to roll a d6 after a Loud or Alarm noise. Zombies only come on a 4, 5 or 6.
Strongholds and the open sea. Noise has no effect.
Every weapon and throwable lists its Noise.
Quiet. All melee weapons, the Hunting Bow, Compound Bow, Crossbow and Nail Gun.
Loud. Every gun without a suppressor, including the Pistol, Revolver, rifles and shotguns.
Alarm. Assault Rifle bursts.
A suppressed gun is Quiet. Save loud guns for emergencies.
Zombies follow sound, and they follow each other. When one starts walking, others near it follow. Over time they gather into big hordes. The big hordes drift with the seasons along the old highways and rivers. The worst is the summer migration across the Great Plains, which the Grange farms fear every year.
Noise can help as well as hurt. Throw a bottle down a side street, set a noise maker, or shout from a rooftop. The noise pulls zombies away from where you want to go. This is called luring. A lure far from you is safe. A lure near you is a mistake.
Do not fight it. A horde cannot be beaten, only escaped.
Go up. Zombies are bad at climbing, except Stalker Zombies.
Go into water. Zombies cannot swim. They sink and walk along the bottom, so move on quickly.
Close doors behind you. Most zombies cannot open a door, but a Brute Zombie breaks through.
Split the horde with a lure, then move while it turns.