On the 400, it sounds like a popping sound, definitely louder. If you have an 07, the gasket was redesigned for 08. I burned through two or three 07 gaskets before I replaced mine with and 08, they are about a 1/4 inch longer and work much better. No problems since.
Btw, the 07 gasket will burn up if you run the bike really fast. After a fast run it seems to burn a small passage, and from that point it just burns up slowly till there's nothing left.
If your bike is an 03-06 model then you might pull the muffler end cap off and check to see if the welds behind it have cracked. Several folks have the end cap melt because a leak behind it caused it to overheat.
I would replace both the gasket at the cylinder head and the one at the junction with the muffler pipe. It's might be a good idea to spray the bolts at both ends with penetrating oil and let the sit overnight before you try to remove them.
12string at first a was going to asked how you liked your Leo Vince Muffler but it's obvious you like it or you wouldn't want to give up your old exhaust manifold.
The older we get the harder if is to find someone to use our old parts.
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