In the first episode of Season 2 of The Walking Dead, the survivors avoid using their guns for fear of attracting large groups of walkers. They take steps to ensure that everyone has hand-to-hand weapons, so they don't have to fire a shot.
Yet later in that same episode, they find a church that has church bells broadcast over loudspeakers connected to a timer, that play several times a day and can be heard from a long distance off.
Is there some reason that walkers would be attracted to gunshots, but not to church bells? What determines what sounds draw their attention?