There is a crouching mechanic in the newer experimental versions. It is toggled with the " key, I believe, by default, which cycles through walk, run, and crouch. You can also bind those movement modes to their own keys.
In older versions, no, there wasn’t any mechanic and really not much you could do. If you could see them, they could mostly see you. It varied based on their sight. If you want exact numbers on a particular creature, you’ll have to find them in the game files. Blind mobs, as far as I’m aware, will detect you if you are adjacent to them.
Loud noises don’t have as much of an impact as you might think on enemy detection, sound seems to work as a ‘move in this general direction and then use your sight to find them’. Blind monsters dependent on hearing were virtually useless.
Scent also plays a role in how you are detected – the general rule is that the longer you’re in a place, the more likely it is that your scent/sounds will draw the enemy into spotting you visually.
Sight really is the most important aspect of enemies detecting you. As it should be.
