Finally fixed my exhaust rattle at 3000RPM.Wasn't Cat converter, Wasn't heat shielts, nothing like that. It is actually A-pipe.
A-pipe has extra inner pipe inside and that's where is the problem. End of the A-pipe( piece going into the Cat) get's loose at the end. I weld inner pipe to the outer, that way it would not move and that is it my friends.
Simple as that.

Hope this helped...