Toyota Avensis D4D Knocking Noise

Hi all,

For the last few months car has had a knocking/metallic sound (Sounds like something hitting )inside the engine )When I take my foot off the accelerator, the sound goes away. When the engine is idling at traffic lights, it sounds normal. Noise happen if I accelerate the car. Just to add it's not timing chain, suspension or plastic cover issue. Any ideas, it is driving me mad as all I can hear on the way to work is this knock everytime I accelerate. Any help would be appreciated.