Monsters will always aggro if they see you, even if it's from across the map. In addition, they'll chase after you immediately if they hear you shoot your guns (or punch your fist silently, although having your chainsaw idle doesn't aggro).
There's some variation to the sound check; for most monsters, they hear you, they chase you, and that's that. However, if the monster has the "ambush/deaf" flag checked, they'll stay where they are until they see you, such that they can, well, ambush you. That said, once they've heard you, their sight checks change from just in front of them to all around them, so if you try sneaking up behind an otherwise stationary monster that has the ambush flag and secretly heard you earlier, it'll instantly aggro to you.