It might be the heat shield on top of the cat converter.
On some models there is a tack weld on the front of the shield and a strap on the rear.
I haven't looked yet to see if the NOX has this setup but it would be a place to start and
have a look. If there is a tack weld between the cat converter case and the heat shield and
the tack has failed...it is more than likely to cause a rattle.
Not saying this is your issue...but it's possible.
On some models there is a tack weld on the front of the shield and a strap on the rear.
I haven't looked yet to see if the NOX has this setup but it would be a place to start and
have a look. If there is a tack weld between the cat converter case and the heat shield and
the tack has failed...it is more than likely to cause a rattle.
Not saying this is your issue...but it's possible.