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THere are 2 possible causes for this type of complaint; you may need to do a little investigating to find the actual source of the noise.
(1) The noise could be coming from an exhaust heat shield or other component. When the transmission is shifted into drive, the engine (which is mounted on rubber mounts) shifts position in one direction; when put into reverse, it shifts position in the opposite direction. When in reverse, the change in engine position may be enough to allow the exhaust pipe that is bolted to the engine to shift position enough to contact some other component under the vehicle, which will rpoduce a rattling noise in reverse only. Have a friend sit in the car and put it into reverse while you lie on the ground and watch underneath the car to se if you can determine the exact location the noise is originating from; chances are you will find a netal heat shield that has rusted through and is no longer fastened to the floor properly, or some other loose component.
(2) The noise may be originating inside the transmission itself. Front wheel drive transmissions have a long chain that can contact the inside of the transmission case if it is stretched or there is other internal mechanical wear. While your friend is holding the vehicle in reverse, use a mechanic's stethoscope (about $15 at parts stores) or a long thin screwdriver held to your ear to listen to the transmission. If the noise is originating inside the transmission itself, it will likely need to be disassembled to repair the problem.
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