Loudness to guards is the likelihood of a stationary guard to begin jogging towards Bond's location after Bond opens fire with either a noisy weapon or after repeatedly and rapidly firing with a silenced weapon. The more shots fired in succession by the player, the more likely a guard in the vicinity is to investigate the source of the gunfire.
A quiet weapon such as Throwing Knives, the silenced PP7 or just simply attacking guards while unarmed is either less likely or has no chance of causing guards to investigate. Loud weapons such as the Shotgun, the Rocket Launcher or the Klobb are more likely to cause guards to investgate the noise. Gunfire from other guards will not cause guards to investigate.
Loudness And Guard Respawning
Some stationary guards in various levels use a module in which they will produce a duplicate guard (often with a different head) in place of themselves to jog to bond's location. These guards will continue to respawn until no more gunfire is heard (but will spawn again if noisy gunfire continues) or if the source guard with the respawn module is either killed or seen by the player, causing no more duplicate guards to spawn from them. These guards will not spawn duplicates if the player has seen the source guard from which the duplicates spawn from, and the guard will instead jog to Bond themselves if gunfire is heard. Levels with good examples of this type of behavior include Bunker 2 and Archives.