Quote:
CVC§ 27000 - Horns or Warning Devices
(a) Every motor vehicle, when operated upon a highway, shall be equipped with a horn in good working order and capable of emitting sound audible under normal conditions from a distance of not less than 200 feet, but no horn shall emit an unreasonably loud or harsh sound. An authorized emergency vehicle may be equipped with, and use in conjunction with the siren on that vehicle, an air horn which emits sounds that do not comply with the requirements of this section.
Which means that they are not illegal, but "unreasonably loud or harsh" is at the descretion of the officer. You could always fight it in court especially if it is close to the tock horns decible level.