The siren's pitch of a vehicle become higher as it approaches to you and become lower as it passes your position. This effect is known as the Doppler's effect of sound.

It is associated with the wave nature of light. As the vehicle approaches, the sound waves from its siren are compressed towards the observer, which increases the siren's pitch. As the vehicle recedes, the sounds waves are stretched relative to the observer, which decreases the siren's pitch.