Hi,
I solved the problem!!
Searching on internet I found that tha cause of rattle is the soft junction between the cat and the Y pipe.
when you mount an HFC or a Test pipe, the increased flow of gas, in some circumstances, starts to vibrate the junction.
The solution:
I made a bar that absorbs the vibrations, but also repairs from scratchs on high bumps.
Notes:
the photo is not the last one....
I have removed the band on the right because the junction loss the mobility...
between the bar and the exaust, there is a layer of silicon high temp. i.e.
This absorbs the vibrations of the junction.