Yea sounds just like your heat shield!!!
I just took care of the same thing on mine.
I had the same prob, rattling at idle and then really loud rattle around 3k or so and then it would be gone.
Take a look under your car and look above and below your cat.,
chances are it rusted off at the mount points on one side and is just rattling up and down onto the cat.!
Just unscrew that shield if so and youll be set!