Messing around with NPCs is a time honored tradition in open world games. Never before have I seen it done with a screaming car.
There are 7 unique vehicles hidden on the map in Watch Dogs 2. This one, the Mountain King, screeches when you honk the horn. Murphurf on r/watch_dogs, has discovered that NPCs really do not like this.
The jeep looks like an homage to Jurassic Park, given its color scheme and model, so probably the horn is meant to sound like a dinosaur. In my opinion it’s closer to the sound of someone dying, or perhaps a very unpleasant labor. If you too want terrorize the Bay Area, you can find the car over here: