Have you ever wondered why when you drive in reverse, your car makes that strange, high-pitched whine? Of course you have; you’re a human, or at least something close enough to a human to go on the internet, drive a car, and wonder. There’s actually a simple, satisfying reason why this happens, so let’s kick some ignorance to the curb.
Just in case you’re not familiar with the reverse whine, I found some samples of it online, in these videos of people driving in reverse:
So, we all heard that sound. Why does the car only make that sound in reverse, and not in any of the forward gears? The answer has to do with the gears themselves.
You see, in most transmissions, the forward gears are what’s known as helical gears, which have angled teeth and are in constant mesh with one another. The angles of the teeth keep the teeth in constant contact with one another, which makes the gears quiet.
For reverse gear, most transmissions use spur gears, also known as straight-cut gears. These are used because they’re much cheaper to produce than helical gears, and since a reverse gear actually uses three gears to reverse the direction of the input shaft, that adds up.
More:
http://jalopnik.com/this-is-why-your-car-makes-that-weird-whining-sound-in-1793174286