This two-minute video gives a clear explanation of why cars make a spooling noise when running in reverse.
In my decades of driving cars, never have I ever encountered a vehicle that runs without any sound when in reverse. Even the Honda CR-V, which is among the cars with the smoothest transmissions out there, makes a noise by the time I hit the R button and look into my rear view while moving. All the cars that I have the chance to drive – manual or automatic – always make a spooling sound; the sound is like a machine that's about to blow up.
Turns out, there are good and practical reasons for this. In this episode of Mike's Mechanics on DriveTribe's Youtube channel, the resident engineering geek explains why.
As explained in the video above, reverse gears are made differently than the other gears used while moving forward. That's the main reason why you have to be in complete standstill before shifting into reverse.
Reverse gears are straight-cut gears that resemble a standard sprocket with the teeth pointing straight outwards. Contrastingly, normal gears have a helical form where the teeth are angled in a spiral formation. The latter is made that way for smoother operation.
The noise, on the other hand, is made by the abundance of surface area that makes contact with each other. With the straight-cut gears, there's a massive surface area, almost everything touches, making the sound. It also transfers torque in a straightforward manner, or as Mike put it, "it’s like ripping off a plaster at once."
Straight-cut gears, though cheap and simple to make, isn't usable in other gears because of the noise it makes – unless you're a race car. Since straight-cut gears doesn't create an axial load, race cars use this gear type for a more lightweight and simpler construction.
Source: DriveTribe via Youtube