Roaring noise

Hi, i have 1999 td5 defender, intermittant power loss, it starts ok hot or cold,doesnt seem to have a missfire, but i have changed the injector loom, fitted new injector seals and washers, and genuine new air flow meter, now at higher revs the engine is making a roaring sound louder than usual and when i turn it off there is an audioble pop from the engine[ like uncorking a bottle of wine], i have an idea what this could be but would like some other opinions, so what do you think