The compressed air or gas that exits the muzzle causes a blast in the same way gunpowder does – by compressing the ambient air into a blast wave. Firearms differ in the loudness of their reports proportional to their power and the length of their barrel. A short barrel makes more noise than a long barrel, given the same conditions at firing (same cartridge, same loading, etc.). So it is with airguns.
You want a spring powered 'firearm' if quiet is your goal.