I've had the same exact problem before and it returned 2 years later after I had it fixed. It sounds like a jar of change rolling around and is most predominate at idle.

The first time I took it to the dealer and had it fixed under warranty. They replaced that whole section of the exhaust on the passenger side from the cat all the way to the can along with the O2 sensor. Sure enough as the VX warranty AND the parts warranty ran out, it started happening again.

I live in the north east, so rust is a problem here and is the most likely contributor to the problem.

Since the warranties ran out, I opted to climb up under the VX to investigate. The rattle comes when you tap on the exhaust right where the TOD labeled heat shield is located (directly under passenger seat). I ripped the heat shield off however the noise continues. It sounds as though the cat internals are breaking apart inside the exhaust pipe. It could be the O2 sensor as well so check that first as the O2 sensor will be MUCH cheaper than replacing that section of the exhaust (about $750 just for parts from the dealer!). ;eekr;

Note: Wear protective eye gear before you climb up under there and start tapping around. Chunks of rust in the eye does not feel good at all based on my experience!

I was lucky and managed to buy both these sections of the exhaust (stock) for $650 shipped to my door!!! This was a special deal here from a fellow VX members friend .

Parts listing for both stock sections of the exhaust will run you about $1,400 not including labor according to the manual!

I know this noise is annoying as hell so good luck resolving it. You'll be smiling a lot once it goes away!