1.6GL that rattle extremely when switched off after driving. on 1990 Toyota Corolla

But strangely, when engine is switched off while stepping on the brake, it doesn't rattle that much!

Asked by for the 1990 Toyota Corolla
Have exhaust system checked for a hanger or heat shield problem.
Possible cat. converter breaking up inside. Don't see how stepping on brakes can make any difference while stopped! Is idle speed way to high / fast ?