What is the knocking noise from the back of my car?

what a stupid question you have given no info about anything or what the car is and expect an answer go smoke another joint or something because this is a waste of time

first answer is just someone iggnorant the knocking could be your rear end going out caused by gears slipping if your brake pads are bad your calipers could knock and then so destroy your brake system if your car vibrates at high speed (60<) you could have a drive shaft issue which could cause wear on your rear end and could be causing the knock then again it could be the dead body you forgot to take out