Sound waves can play tricks on your ears in what is known as the Doppler Effect. For example, a car approaching you has a high sound pitch that lowers once it has driven past you, despite the car's noise production not changing at all. If you were sitting in the vehicle, you wouldn't notice any change in the car's noise at all. The car's sound wavelength frequencies stay the same throughout approaching and passing you.
However, the car's speed as it moves towards you makes the sound waves hit your ear at a faster rate or frequency than the vehicle is making them. That makes the engine's pitch sound higher. The opposite occurs once the car passes you — the sound waves come to your ear more slowly and at a lower frequency, making it sound lower.