X2 on the exhaust bolts. Did lots of them at the dealer and in my own shop now. I'd recommend sticking with the factory bolts. The washer made on it is cone shaped to allow expansion and contraction of the manifold. BTW, this is what breaks the bolts. Stick with the OE gasket and I wouldn't coat it. The material on the gaskets' head side is designed for aluminum and the manifold side is steal shim crush. Never seen a factory gasket leak until the bolt broke.

With all that said, if it did it since new, it's probably the lifters and/or injectors. The lifters all have a slight tick. The injectors are just loud. The 6.1 is probably the loudest because of the little covers from the coils that don't quite cover the injector. They also have none of the sound absorbing foam like the 5.7 engine covers. I take a new under manifold foam insulator and cut it to fit under the little covers and around the injectors. Speaking of the insulator, you said you were carb'd. Not sure if you removed the insulator, but if so, do you have room under the intake to put the factory foam insulator back in? This may not cure it 100%, but will make a noticeable difference.