Rattling Noise from Catalyetic Converts - advice needed

[2001saxster]

I took my 01' Boxster S in to a local independent shop the other day for an oil change. Asked to check on a sort of rattling sound I was hearing from the back of the car. Was hoping it was just a loose heat shield, but alas it appears the rattling is coming from the catalytic converters...specifically the rear "secondary cats".

I have a few questions about this that I was hoping someone with more knowledge than me might be able to shed some light on.

First, what would be causing the rattling?

Second, is the rattling a sign that the converter has already completely failed, about to fail, or is it operating at reduced capacity?

Lastly, what would happen if it does fail completely? Is there risk of damage to other components, or will it just not process the exhaust up to emissions standards.

Regarding the last question, I know this is going to be a pricey repair and money is tight at the moment. Unfortunately, this car is also my sole daily driver so I don't have the option to just not use it for a little while. I can live with the rattle for a while so long as I'm not risking damage to other systems.

Any advice would be greatly appreciated.