Beyond Enemy Lines utilizes a movement based noise calculation system. Each move or action will result in a noise with a different intensity, calculated from your movement. Not even walking or shooting a gun produces a noise, even reloading the gun, opening a door or toggle a switch cause noises. It depends on you how you use the noise to your advantage, whistle to lure guards into your direction or distract them with other noises. Act carefully, the enemy will hear you!Read more about the Noise Calculition System on the official Wiki.