That make sense for me, because when they see somebody along the way to target, they switch to first visible target, doesn't matter who is shooting, they are following sound. The infected are not very intelligent.

I think the issue is more, the infected have super human sight. Also super hearing on bullet impact.. When the gun is louder than the impact the infected should not be moving towards the quieter sound even though it's closer.

I've seen them aggro on streamers laying in the grass mostly behind a corner overlooking shots.
Infected is drawn into the area by shots, does a U-Turn! and aggro right on the player 98% covered..

It absolutely makes sense, that they attach to a target along the way to the shooter. BUT: Make the vision field of the Z's smaller as soon as they react to a sound. It makes no sense for them to turn around 179° because there is someone closer. Also, when shooter and target are only 20-40 m appart from each other, the gun should always be the louder signal source for the Z's. I've had situations, where I got shot at, the shooter was roughly 100 m from me away, there were Z's between him and me, all closer to him, still I had to manage with them and I didn't even shoot back. Z's are stupid, you are right, but they need to be the right kind of stupid, not "we run into the complete opposite direction of the shot" stupid.
When I make no noise at all and I am prone in the grass, spectating a gun fight, Z's should NEVER attach to me, that makes absolutely no sense to me. They should just ignore the player for that case. Can't you flag a shooter as "hot target" and a player who was laying prone by the time of the shot, a "cold target", or invisible target? There should be a weighting for possible targets, depending on their stance, movement speed and "most recent" activity.

@jamue Yes, U-turn have no sense when you laying hidden in grass, of course. You can't mark shooter(no sense), but you can mark the place where the sound came from. Infected shouldn't know who was the shooter until he didn't see him. It's good when infected responds to sounds and visual perception only. This is how they working now. I enjoy it when, you shoot in the city and have time to hide until the infected come and can see you. I remember those times, and I'm glad they're gone when the infected would attack the nearest target only. However, in some cases a U-turn can occur today. I agree that this inaccuracy can be very frustrating, especially when you are trying to remain a hidden spectator. :(