is it when u turn, striaght line, braking or all the time? id jack up the car and take off the tire. check out your shock and sping and inspect the bushings like jordanhxc said. is the car lowered or have any other suspension mods to it? im not to familiar with that year civic, does it have a rear sway bay? a friend of mine had a busted endlink on his teg and it made a terrible thumping noise when he turned kind of hard or had peopl in the back seat. take a few pics if u have a camera and people can help u out a lil better.