What is the noise some cars make when reversing?

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I always wondered why it sounds different from a “normal” forward drive.

In: Technology
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Most forward driven gears are helical, so they mesh together on an angle. They’re more expensive to produce and require synchronizers, but they are quieter.

Usually reverse gear is a straight cut (spur) gear, which is much cheaper to produce and doesn’t require a synchronizer. The trade off is they are much nosier and produce that whining sound.

[Helical vs Spur gear](https://i.kinja-img.com/gawker-media/image/upload/fbmmlsperzp2tttsiffv.png)