Movement, weapons, and utility all generate audible cues that can alert your opponents. It’s important to be aware of the sound cues that you make, while also using enemy noises against them. Running, jumping, and falling from a high enough distance all make noise.
However, crouching, walking, and crouch-walking are silent. Firing a gun, reloading, zooming in with a scope, and switching between Automatic and Burst fire modes are all audible, but cycling through your equipped weapons is inaudible. When a grenade explodes or hits a surface it will make a noise, but throwing a grenade makes no noise.