Is it a tinny metallic sound? It is probably just a loose heat shield - totally benign. You can just ignore it or crawl under their and cut the thing off.

Nissan in their infinite wisdom put heat shields all over the place down there...any one of them can rust out and in fact most of them do. When you took it back to the dealer to have it checked you have to wonder if he checked them all or just glanced at the ones around the cats and responded yup they're fine.

Point being I've pulled quite a few of them off over the years (as have others) as they rust out around the mounting bolts. The rings at the flexible connection just behind the trans output shaft tend to go first, then the pass side pipe, followed by the drivers side....

Pull the thing in the garage if you have one, put on an old shirt, grab a flashlight and crawl under there. Check each and every heat shield top and bottom from the manifolds back. Try to wiggle them pry a little with a screwdriver right near the mounting bolts. Don't forget those two rings at the flexible (braided) pipe near the trans. output shaft. They shouldn't move at all. If they do move or are just bouncing around loose on the pipe as mine were you found the problem.

Good Luck
-Saturday