It sounded like from one of the brakes (or in that area), definitly the undercarriage in the back. The check engine light came on like a second after the noise... I have no idea what could be wrong... i looked under the area and saw nothing loose or anything out of the ordinary...

Just bought a 2002 S80 2.9, when I started backing up I heard a loud clunking noise from the rear of the car and the check engine light came on... any suggestions...