Your comments about a rattling around sound in the engine bay and loss of power made me think of pre-cats. This is exactly what happened to mine, even though it was a v6. The pre-cats crumbled and blew debris down the exhaust and clogged up the main cat. The remaining chunks of pre-cat catalyst were bouncing around in the exhaust manifolds. Not sure if the zetec is prone to the same failure, but try listening to the pre-cats with a stethescope while someone revs the engine to see if that's where the sound is coming from. You should here something like BBs in a coffee can when 'listening' to the handle of a big screwdriver with the blade held to the exhaust manifold. (Old indian trick...)
