Almost all actions generate some level of Noise, which can be heard and subsequently investigated by nearby enemy units.
A Brigador should be conscious of the noise they are making, as building destruction, explosions, weapons fire and impact can put enemies in the surrounding area on alert to investigate the source. Some noises are louder than others, alerting enemies in a wider range.
Each weapon has a noise value for its initial firing as well as the impact of its shot. Some weapons, such as the Black Hand or the Crambo, are near-silent and allow for stealth runs in powersuits or other small vehicles. Others, such as the Heavy-class 'Bishop' Cannon, have deafening reports and will alert enemies far off-screen to your presence the moment they're fired. Horns all have the same noise value, attracting enemies near the top and bottom of the screen, but not far left and right (on centered view, in 16:9 screen format).
With some forethought, enemy units can be manipulated with noise; firing loud weaponry, or sounding off a horn, and then quickly changing positions and ducking behind cover before they converge can pull opposition away from Ammo Depots or mission-critical structures, or bring several scattered enemies together to one place for easy mortaring.