On the mk2, the cat is on the downpipe i.e just after the exhaust manifold, in the bottom of the engine bay.
The rattle sounds like a knackered centre or rear silencer box, caused by the baffles inside coming apart and rattling around inside.
Either that or, if you had a replacement exhaust that was not the original fiat part, the back box can foul on the rear axle.
Get under and move the exhaust and see where its coming from. You may have only a rear silencer or a centre one as well, if either is making the noise, get it replaced.