curtis ward

i wondered if anyone could help,
recently had 4 new injectors put in my van 2 months ago,
3 days ago my van starts making a clacking noise coming from the heart of the engine,
took it back to garage that done the 4 injectors as they under warranty, they have put a new injector in cylinder 4 but it hasnt made a difference,
they are now going to check the wiring harness however i doubt that it will be that,
they have removed the rocker and looked at the cams but haven't seen anything obvious so the next step is a compression test,
also the reason i am on bere is i have very little faith in this mechanic and i only went back there is becaus ei thought it was a faulty injector,
any ideas would be really appreciated,
311 cdi mercedes sprinter 2010