A failing cat-con gasket will tend to get loud and stay loud rather than rattle. After all , it's a crushable lead gasket held in by spring bolts to form a flex joint.

As you know already, the infamous OEM exhaust manifold rattle happens because the weld holding the inner manifold pipe in place fails.

The rpm at which the rattle is loudest can vary depending on where the weld fails. Unless you plan to replace the OEM manifold with an aftermarket exhaust header, the cheapest solution is to have the inner pipe re-welded by a competent exhaust shop.