I guess it is the Pre-Catalytic Converter.
My car has exactly the same symptom except
it's a V6 duratec.
The pre-catalytic converter is blocked by
the disintegrated catalyst, so the exhausted
gas leaks from the pipe and is absorbed
via the ventilating system to your car.
I even observed this one time I crawled under
the car with the engine running. Some fume
came out from the exhaustion pipe, just like
the fume when carbon is not sufficiently burnt
in the stove.
Blocked pre-cat. convt. also explains why the
acceleration is so sluggish when starting up,
and the engine utters a loud noise.
Hope this helps a little
