It felt a hell of a lot like there was gas escaping at the base of the cat, above the outlet port.
Visually I cannot see any cracks or anything like that.
And it is quite possible what I felt was just gas jetting out from the top and travelling around the cowling.
So... clean it all up, new gaskets, refit, and hope... or try to test the unit for leaks. I will need to make some blanking plates, and a way to pressurise it. (I do have parts/tools/stuff I could do this with.)
