Have you tried opening the fuel filler cap ? as a vacuum in the tank can cause this sort of problem. Doesnt happen on short journeys because vacuum doesnt become strong enough on short trips. Causes a Whooshing noise when the vacuum is released, stopping allows the condition to recover slowly.

Just a short intro, as a long time visitor I joined the forum a few days ago. I've recently aquired my second TD4 Freelander, I had one about 12 months ago but was persuaded to part with it ( bad move), however I needed another motor with slightly better fuel consumption than my 96' 300TDI...