The only info i got about the noise mechanic was from reddit user "zeidrich" , he essentially said that each unit generates different "activity" levels on the tile per attack, which diminishes over time, but if a unit shoots faster (e.g. ranger veteran, shooting twice as fast), it logically should attract alot more zombies. Though i have to say, i have no proof of this in any way, i just believe him on this right now as i dont have a better explanation or way of finding one, and it seems about right. The reddit thread talking about this: https://www.reddit.com/r/TheyAreBillions/comments/7n0p97/can_the_noise_system_be_explained_better/
Every shot makes a certain amount of noise, and since veterans have a higher rate of fire, they generate a little more noise. However, a veteran Ranger kills a basic zombie in 3 shots instead of 4, so that's less "noise per kill".