When turbo inlet sucks pipe there is no airflow, that's why intercooler to manifold hose is sucked by engine.
I've had this problem on at least 3 pre lift P38's, replacing turbo inlet elbow pipe cured the problem completly.
I'ts strange that Land Rover didn't recall vehicles with this issue(but they knew about the problem, hence why it got upgrade to plastic elbow with facelift), it's very dangerous in for eg overtake situation, engine suddenly looses all power forcing you to stop the engine. In very hot conditions this rubber elbow closes turbo inlet completly.