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!

1 answer
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 ?