Noisy Ford Fiesta 1998 1.25cc Engine?

Hi

When I drive my fiesta, each time I change gear and when the engine revs fall slightly into the higher gear, the engine makes a load knocking noise. This only happens when I release pressure from the accelerator.
It also makes this loud noise constantly when driving on the motorway doing a steady speed. The knocking dissappears when I accelerate. It almost sounds like the piston tapping on the side of the engine, becuase the higher the cars revs the higher the freqency of the knocking noise.

I have replaced the oil with a thick 5W-40W, which did help a bit but the knocking is still loud.

Hope you can help.