Why does your car make loud noise when you reverse?

The lower gear ratio of the reverse gear causes the "loud" noise. The final drive section of front wheel drive vehicles is right in front of the driver. Due to the proximity, the lower ratio (which provides power/torque and not speed) and fact that the gears in reverse arn't used much, therefore not as polished, they are a bit more audible. The reverse planetary is also going in a direction opposite to what it usually runs. that's my theory anyway.