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 seven 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 colour 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 labour. If you too want terrorise the Bay Area, you can find the car over here: