Yes, cat and catalytic converter are referring to the same thing. Unfortunately, we have 3 cats on our cars. If you've replaced the y-pipe and "main" catalytic converter, Ford wont cover the add-on parts under the normal, government mandated, 80K mile emissions warranty. If the SHO cat (under the car) is rattling, then you'll have to work with SHO for a warranty replacement. It might just be a loose heat shield which a muffler place could weld up for like $10.
Now for the other cats... The V6 also has 2 "pre-cats" These are built into each exhaust manifold, between the cylinder heads and the Y-pipe. If these pre-cats are rattling, BE CAREFUL! The stuff rattling around could get sucked into the engne and cause major damage. The pre-cats are also covered by the 80K mile emmision warranty, but the dealer will probably give you a hard time about covering them on warranty because of the custom Y pipe. (It's worth a try though....) These are real expensive to replace ($1000+) and many of us just "gut" them and remove the ceramic matrix thats inside the manifolds. (Search on mystery mod)
To see if your pre-cats are falling apart and rattling, try using a big screwdriver as a stethescope. Listen to the handle and put the blade up against the front bank exhaust manifold while the engine is running.
