Loud is relative.
Compared to firearms, and even to many airguns, they are not. Usually, the sound of the motor or the action cycling is the loudest thing you hear when shooting an airsoft gun. These noises can be compared to the sound of a cordless drill.
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Most airsoft guns shoot at less FPS (Feet per second) than a real gun. Also, the pellets they shoot are far less dense and would take a higher speed to kill than a bullet from a firearm would. They can possibly severely injure and/or kill if you shoot them into your eyes, so if you plan on using one, be sure everyone within the close vicinity is wearing some sort of thick plastic eye protection. The worst you could do with an airsoft is probably kill a bird or another small animal, but that is animal cruelty and you can be arrested for doing so.
So, long story short, as long as you where eye protection, and don't aim at anyone's face, no, the worst they can do is leave a welt if you are shot.
It depends what air-soft gun you are talking about, what rate of fire it is, and what is used to power it.
(removed this part because i thought you where talking about nerf...fail)
That depends on what powers it electric guns are a usually fairly quiet, spring guns are about the same, C02 is kind of loud, and I'm not sure about green gas.