Tapping/ticking noise on acceleration?

[spolky]

Over the last few weeks my 2.0 TDCi has started making some unusual noises when accelerating - but only when the car is moving/engine under load. When stationary there is the slightest hint of ticking or tapping the second you touch the throttle but then it disappears and the engine revs smoothly and quietly. But out on the road the noise (or noises as there are a couple of distinct ones) is extrememly noticeable and only disappears when you ease off the throttle. I'd describe the noise as lighter and softer than normal diesel clattering, but very loud. Acceleration feels slightly down and fuel consumption is slightly up. Anybody else experienced this and know where I should start looking?