Travel Light is a gear perk that greatly cuts your movement noise while your weapon is holstered. It lets you move quietly through an area with your hands free, then draw when it is time to strike.
With your weapon put away, you make far less noise when moving. You can approach guards for takedowns without drawing their attention.
Wear the Caveira Fatigues, Photojournalist Pants or Standard Police Pants. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Stalker reduces noise while crouched, and Quiet Confidence helps after takedowns.