It would be a surprise to me if a cat failure would result in any noises. There are no moving parts in a cat.
Typical symptoms of a cat getting clogged is that the engine sort of 'hits a wall' where it just won't produce any power above a certain RPM.
My first thought would be that it has a loose and noisy heat shield on it. This can make a terrible racket, but it is usually easy to fix.
1986 300E 5-Speed 240k mi.