Possible manifold leak
 

Hey all, so I am experiencing a possible exhaust leak coming from the header/manifold area but I wanted some insight before I dig deeper. On start up and only when the motor is cold (ie. sitting over night) I get a slightly loud tap for maybe 30seconds? Then it sounds perfectly fine afterwards. I have a pending code for p0420 with brand new NTK o2’s and redid my exhaust all the way back. No leaks coming from the exhaust itself and on occasion I’ll get a pretty strong whiff of gas and I kinda feel like I’m getting crappy power. It is a 96 4.0 with a 99+ manifold and I managed to score a $4.00 ATP SS header from rockauto. New 12 hole injectors from k suspension as well as aluminum fuel rail, bbk 62mm throttle body, cps, tps, iac and map sensor. On top of that it also has a brand new Clearwater head I put on. Also does anyone know the size of the little flange gasket that goes on the header and recommendations on manifold gaskets? I am also aware that these things have noisy valve train and lifters but I’m just ruling things out. In my experience….lifter tap goes away when the motor warms up but this morning it was at 60F when the tap stopped. Thanks!