MilesP: I would drop the exhaust pipe and look at the gasket to see if you see black soot
where the exhaust gasses may be leaking across the gasket. If not leaking there, pull the
exhaust manifold loose and then look at the exhaust manifold gasket for black soot streaks
across the gasket material. I had to put a double gasket on part of the exhaust manifold because cylinders 3 and 4 were leaking because the manifold was warped. I also had to put two exhaust pipe gaskets on to stop another leak, because the flange on the exhaust
manifold was corroded. If the gaskets show no sign of leakage, you may have a small
crack in the manifold.