Klaxon

A horn is a sound-making device installed on motor vehicles, trains, boats, and other types of vehicles. The sound it makes usually resembles a "honk" or a "beep". The driver uses the horn to warn others of the vehicle's presence or approach, or to call others' attention to a hazard. Motor vehicles, ships and trains are required by law in some countries to have horns. Public transit vehicles and even bicycles are also legally required to have an audible warning device in many areas.