I know this is pure necromancy, but i'm experiencing the exact same problem as OP was. MXP SP rest of the exhaust is stock, after the exhaust heats up, at idle/standing still, there is a very loud intermittent pop/clang noise coming from the exhaust area that subsides as it cools down. The clanging has its own rhythm independent from the engine rpm or other wise. No issues with clearances or contact points. It's harder to isolate it because by the time I get it up in the air the exhaust has cooled down and the noise disappears or becomes hidden by the regular exhaust pinging.
Any one else experience this?
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CSG Mike have you guys ever encountered this with any of your MXP products?