What could be the reason for a ticking noise from a Citroen C3 HDI diesel engine?

It could be one of the injector fire seals.

There is a forum post dedicated to the Citroen C3 injector fire seals here:

http://citroenc3owners.com/viewtopic.php?f=6&t=12

and a youtube video demonstrating the sound of a leaking fire seal (ticking or chuffing sound) here:

http://www.youtube.com/watch?v=UgZ-B0Ccafg

or, it could be the crank pulley.

There is a video of the sound a worn crank pulley makes and information about the parts required to fix it at

http://citroenc3owners.com/diesel-citroen-c3/citroen-c3-2002-1-4-hdi-engine-makes-loud-ticking-noise-t290.html