Something like this would just be used as a way to get large amounts of XP very quickly meaning players "rush" through the game without really playing it.
Didn't someone just suggested something similar last week? I'll use my same response from that thread.
Im not talking about having a constant rush in the missions, just an increased chance for them to happen. Sometimes I have zero rushes in a mission. The book's increase would likely not be 50% or something massive like that, but 10-20 maybe. If we're worried about XP abuse, we can even add a small reduction to XP as well.
Why not equip a Hardcore book with a They live again?
You may dread a zombie rush
I do that....and yeah, I would dread them then.
Based on personal experience from running 2-man teams, one with melee and a silenced SCAR and one with melee and a "noisy" SCAR, the team with "noisy" SCAR more often than not was seeing a significant increase in zombies over the team with with the quiet weapons.
My gut says that noise works for zombie spawns like shiny/looter works for quality items -- it only increases the chance of the "desired" result. The rate of zombie spawn on missions is not fixed, there's definitely a randomness to it. Sometimes you'll get a steady trickle of zombies, others' you'll go whole minutes without seeing one. Usually (but not always) when I'm getting a lot of zombies on missions it's when I have noisy weapons equipped. Likewise when I'm not seeing any zombies it's usually (but not always) when I have quiet weapons equipped.
Zombie rushes (in my experience) seem to follow similar behavior. In otherwords, noisy weapons only increase the chance of zombie rushes, but do not guarantee them.
Side note: Personally I'd never use chainsaws (even if I hadn't out leveled them by now). Unique and rare versions aside, they're noisy AND slow, a bad combination in my honest opinion. You can be slow, or you can be noisy, you can't be both, at least that's how I see it.
The reason you see more zombies with noisy weapons is likely because the ones on the map will be drawn to you and you will take them out quicker which will trigger the next wave sooner. When using suppressed weapons, you will likely not draw all of the zombies on the map to your location which would increase the time between spawns and, sometimes, in my experience, stop them from being triggered at all until the other zombies are mopped up.