The rattle you hear is likely the broken bits of cat gut. These cats, unlike those that Stijn uses, often have a ceramic honeycomb internal structure that is coated with palladium, rhodium, or platnum (which is why they get stolen). As they age, they can crack, breakdown, etc, leaving you with crumbled honeycomb rattling around.
http://en.wikipedia.org/wiki/Catalytic_converter
Any quality aftermarket cat should be fine to replace the OEM.