Your next question, of course: Why? I've heard a number of interesting explanations over the years, but the real answer is in the gears. Specifically the way the teeth in the reverse gear are shaped. The forward gears in your transmission are helically shaped, meaning the individual teeth on the gear are curved rather than straight (see photo) but reverse has straight cut teeth.
Why is straight noisier than helical? A helical gear's teeth are always in contact with the next gear thanks to their shape. Picture two straight gears moving very slowly. There's a split second that you'll see space between the teeth. The continuous tiny tapping of the gears remaking contact with each other creates the whine you hear. Since a helical gear has long, curved teeth, the point at which two gears meet is never out of contact, so no tiny taps, and no noise.