California Code, Vehicle Code - VEH § 27150.3

(a) A person may not modify the exhaust system of a motor vehicle with a whistle-tip.

(b) A person may not operate a motor vehicle if that vehicle's exhaust system is modified in violation of subdivision (a).

(c) A person may not engage in the business of installing a whistle-tip onto a motor vehicle's exhaust system.

(d) For purposes of subdivisions (a) and (c), a “whistle-tip” is a device that is applied to, or is a modification of, a motor vehicle's exhaust pipe for the sole purpose of creating a high-pitched or shrieking noise when the motor vehicle is operated.