i had 1 of my cats go, and also the gasket between the manifold and downpipes was blowing, made the engine sound terrible and i worried it was gonna be a big job, suprising what a difference sorting the gasket made to the sound of the truck. wait till the engine/exhaust are cold, start it up and quickly before the pipes heat up start feeling for escaping gasses, start at the manifold and work backwards as its the front end that will get hot (quickly) first.