Neil
I am not an expert by any means, as you well know, but I have been told several times that the worst thing for a cat is driving through water.
Apparently the platinum catalyst is mounted on a ceramic former inside the metal case. The catalyst runs very hot and the thermal shock of cold water on the metal case being tranferred through to the ceramic can apparently cause it to crack and once it starts going...............
The result is the ratlle which I have, although mine still works fine, just sounds awful.