I had a cat start rattling on my 5.2 and after changing the catalytic converter I checked it with my smoke checker a redline tech plus..
This is on cold exhaust, nice and tight.
I found leaks at the metal on metal fittings at the cat to muffler conections and at the bypass valve. No leaks at my donuts all new ones and no header leaks.
I took it apart and looked it all over and now back together its still leaking at these spots.
I made sure they are flat flush double checked it all.
Please Tell me when its cold that smoke leaking at these metal to metal conections are cceptable because the little bit of brain i have says yes when it heats up it seals those areas better.
I was thinking about checking it while runing like i did aircraft and feel.for any exhaust leaks as you find them pretty quickly.