system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When you turn on the turn signal in a car, why does it sound like the clicking noise is alternating between a higher and lower sound? Is it really alternating or is it just the brain creating an audio illusion?
gpt
Turn signals originally were controlled by electromechanical relay under the dash. The clicking sound was that of the relay opening and closing based on a timing circuit. The slight differences in sound was the difference in sound made by the relay when it was opening or closing.

Modern cars don't really need the mechanical circuit. The could be made completely silent by using different components (physical relay may actually be cheaper though). However, people have come to expect to hear the sound of a turn signal. Some car manufactures will actually tailor the sound to how they want. Along, a real sound is a good reminder that you have the turn signal on (or left it on by accident).