I imagine implementing a simple three level aggro system to the infected AI that represents an animal like "excitement" of how close they think they are to catching you. It is based on the Zs closest travel distance to the survivor. If a Z spots you on a rooftop or or outside of 50 meters (extremely long travel path to reach you, trough building, up ladders) they will go into the lowest tier of aggro, where they "moan" (and like Moriarti suggested, potentially alert other zombies to close in), shamble slowly towards you and eventually start "looking" for a path to reach you (start traveling the closest path to your position, this might result in them loosing aggro if you stay passive). The mid tier aggro would be Zs spotting you at a distance but with less than a 30-40 m path to you, where they would start a aggressive shamble/jog towards to, hissing and screaming (hopper aggro speed). If a Z spots you or reaches you on level ground at a distance of 10 meters or less, they will hit the highest tier of aggro, where they go absolutely mental and stumble head over heels towards to (foooood!). If a Z spots you within this distance they would quickly cycle trough the lower tiers, giving you a split second to react before they come at you full speed. I imagine the final tier movement looking something like the first zombie seen here: http://youtu.be/KYbQ88oFuxM?t=1m16s If you loose a Z, using standard Z loosing tactics, they would go into the search mode already implemented before going back to normal. Reactions to gun fire and other noises would be volume based, triggering one of the three levels of aggro towards the spot where the sound originated from (since the infected are said to have superior hearing). A Lee Enfield going off close to a zombie is an instant tier 3 aggro on the shooters position. TL:DR, Three aggro speeds triggered at >50 meters, 50-10 meters and <10 meters of travel distance to a player. Gun fire and noise aggro to the spot where the sound originated from, aggro speed based on loudness of sound.