I keep having the same issue with an exhaust leak from the flanges that attach the y pipe into the cats.
I've tried new gaskets. I also tried high-temp gasket maker.
Anyways these worked for some time, but the issue kept coming back.

I'm sorta fed up and thinking about putting 2"-3"(length) inch pipe inside the exhaust flanges then bolting them up to finally solving this issue. My concerns are when the exhaust expands and compresses due to cold/heat as I drive it all year round in Canada... what type of 2"-3" pipe material should I use for this job?

 

Probably a broken hanger somewhere that's putting too much stress on that point.

 

Not the most elegant solution, but I've used small C-clamps to close up flange gaps with relative success. I also tried high temp o-ring gaskets like , but they only seemed to work for week or two before the sound came back and the gasket either burnt up or took up residence somewhere further down stream in the exhaust.