1990 Geo prism, mileage: 121,220. There is an unidentified rattle in the underbody towards the rear of the car which is not always present, but when it is, it is loud as if the exhaust pipe was bumping the underbody. I replaced the exhaust system from the flanged part behind the catalytic converter and assume that is the replacement pipe which is knocking on something. While the metal hangers on the replacement match up exactly with the rubber fittings on the car, the bends in the two are not identical. Still, this doesn't seem to impede anything. I have secured the metal hangers which are attached to the replacement exhaust pipe with metal hose clamps on either side, but the sound is still there. It is not the same sound when I am driving as it is, when you physically shake the pipe to make it purposely hit the underbody. It also stops if I stand behind the passenger seat. Getting out of the car and visually looking at the tailpipe, everything seems to be stable. Are there other things I should be looking at besides the exhaust? I am really frustrated because I am sure I installed the aftermarket pipe properly. Any advice is appreciated.