Seems as if it would be a cracked manifold or worn out gasket. Can you hear the engine ticking while under the hood from the exhaust manifold side? If yes then you have an exhaust leak somewhere. As far as fumes entering the cabin, again logical because I had a small oil leak after replacing my valve cover gaskets. I though all was well until I could smell a strong burning smell after driving. Open the hood and its smoking lightly on the right bank. I let it cool down, run my hand around back and felt the gasket warped and spilling oil. I used a mirror and thought I felt it correctly during install, but no. The thing is I couldn't understand why the smell would reach the cabin if the odor is under the hood. So my point is is that it is possible to get fumes in the cabin with the recirc on the a/c controls set to auto. If you are getting an exhaust smell its likely the manifold or gasket. The 3 liter diesel is no different from the gas 8 in terms of what exhaust gas smells like in the cabin, so that would be my guess.