A warning device including a metal housing (1) sealed with a flexible metal membrane (2) secured to a plunger (6). The inside of the housing (1) comprises a coil (5) with an air gap into which the plunger (6) may be inserted, and a breaker (7) having an upper arm (8) and a lower arm (9) with mutually contacting contact portions (11, 12). The breaker is attached to the housing (1) by means of a rivet (13) and a ring-shaped varistor (21) around the rivet is fitted between the arms (8, 9) of the breaker (7). A shoulder on the plunger (6) is engageable with the contact portion (12) of one arm (9) and supports an insulating washer (23). The interfering signal generated by the electric arc whenever the contact portions (11, 12) are opened may thus be controlled. In use, said audible warning device has no adverse effect on the electronic equipment of the vehicle.