no i think it's leaking in your spark plug cyl because soungs like blowen head gasket or cracked head thats the only way it can get into your exaust. unless by chance you have a freezze plug with hole in it and going threw exaust somehow only way that could be is hole in exaust is at same place but vey unlikley.and if it was heater core it would fog up your windows when your driving it.i say head gasket.