The fact that you lost torque and your getting this noise from the exhaust it's the cat going bad. That rattling noise you hear is the honeycomb that has broken apart inside and is restricting the flow of exhaust. This can cause unrepairable power loss to the engine if left unfixed. I would change the cat asap. I recently had a similar problem with my car but I also got a horrible smell from the exhaust at times. I didn't have the money to change the cat so I just hollowed it for a temp. solution.