Don't worry, it's not that loud, especially on our little four banger engines. It won't be any louder at idle than your stock box, at least it won't be noticable. When driving, especially at wide open throttle, the thing really comes to life and growls.
The only time your neighbors might hear it is when you shut your car off, it will make a sucking sound. Still not loud enough to be a bother. Tell your neighbors to kiss your ass, because the noise that would resonate from your car with a CAI
will be more than legal.