Sometimes it is difficult to isolate a sound. They tend to echo through the body of the car. You may just have a header gasket leak. If a header bolt comes loose, it may be enough to allow a portion of the gasket to get blown away.
USMC302 is correct that Mustang headers do not need gaskets between the collector and the h-pipe. They use a spherical joint. You just need to make sure that you torque the header to h-pipe bolts correctly. Also make sure that you have a spring washer between the nut and the h-pipe flange so that it doesn't come loose. If it still leaks, something must be defective.
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