This circuit simulates a chime similar to the sound many cars make when the keys are left in the ignition. The bottom two gates form a squarewave audio oscillator that drives the base of the 2N4401, turning it on and off at an audio rate. source

This circuit is intended for children fun, and can be installed on bicycles, battery powered cars and motorcycles, but also on models and various games and toys. The typical dual-tone sound of Police or Fire-brigade cars is generated, by the oscillation.source

This is a simple siren circuit but powerful.The no need ICs and requires 12V Battery operation.The stand-by current drawing is zero.source

This siren circuit simulates police, fire or other emergency sirens that produce an up and down wail. The heart of the circuit is the two transistor flasher with frequency modulation applied to the base of the first transistor. source

This circuit produces a sound similar to a factory siren.It makes use of a 555 timer Ic used as an astable multivibrator of a center frequency of about 300Hz.The frequency is controlled by the pin 5 of the IC. When the supply is switched ON, the capacitor charges slowly and this alters the voltage at pin 5 of the IC hence the frequenct gradually increases.source