Related to this question, firearms make different levels of noise depending on the firearm in question and the choice of ammunition.
What factors cause one firearm to be louder than another?
There are a number of different factors that would result in less noise,
Subsonic ammunition as bullets that break the sound barrier create a sonic boom, while subsonic ammo does not. (depends on the cartridge)
Single shot actions, semi-autos have moving pieces that make plenty of noise in their own right and they open the action to load the next round and noise comes from that as well.
Barrel attachment: A muzzle brake will increase the perceived noise (some up to 9 dB) and reduce the recoil while a suppressor will reduce the noise the shooter hears.
Some of these make more of a difference than others, with cartridge choice having the biggest impact.
Additionally to the previous answer, gunpowder quality (including pressing quality, corning, the size of the granules, etc.) is one of the factors affects on louderness and power.