My car was making a rattling noise when driving with very low RPM and slow, or very high RPM and fast. When cruising it was OK. Drove car to the mechanic and he said that cat is broken. He replaced the cat and now everything is OK. Anyway I looked under the car at what he did and checked in my cars manual, and noticed that there should be a heat shield over the cat. I googled and somewhere it said that without heat shield over the cat the body of car can get so hot you can feel the heat in the cabin on the floors and its not OK.
So did he remove the heat shield? Do my car need a cat heat shield? My car is Renault Clio mk.2
I also suspect that the rattling noise came from heat shield and he just took it off and overcharge me for new cat.