Exhaust system original 2.0 V6

[Costa Fortuna]

So I think I have the cat rattle feature - whilst changing the oil I decided to investigate the noise.

I'm pretty sure it is the cat because a tap with the end of a lump hammer handle produces a sound like there is something loose.

But here's the thing - working from the rear of the car there is the large box under the boot floor, a joint just rear of the axle line and then that's it - no more joins until the headers. There is a triangular box where the rear seat passengers feet would be, another round one under the centre console, then about 9" in front of that another circular box, then a flexi (which is a concertina type not woven), the cat then up to the manifolds.

There does not appear to be a downstream sensor just one on each downpipe. So how does that work?

Must surely be the original from the factory what with only one joint - I would safely assume the rear box has been changed at some point.

The car is a 2000 flavour manual.

At some stage this cat is going to need replacing - aftermarket ones seem to show a downstream sensor hole and just look different generally and Rimmers looks about right in their illustration but the aftermarket replacement looks the same as the ones available on the on the interweb.

Any advice/previous experience shared would be gratefully received, many thanks in advance.