The resonant frequency of the exhaust pipe, often tuned by engineers to sound aggressive or "musical."
In media and sound design, this effect is often used as a shorthand for . race_car_passing_sound_effect
From the driver's perspective, there is no Doppler shift . The driver moves with the sound source, hearing a constant, deafening mechanical drone rather than the "passing" effect. A Matter of Life and Death - NASA The resonant frequency of the exhaust pipe, often