Jalopnik:

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.

I am mechanically incompetent and, while I’ve noticed this sound, I had no idea what created it. The explanation is actually kind of interesting.