worn cams can cause various different symptoms and a loud clicking noise from the engine is one of them.
best way to check is to take of the cam cover and inspect the followers for wear, they should be smooth, not have any pitting on them. then check the cam lobes, these should be smooth and not have any flat spots on them
to take the cam cover of you need a 10mm socket and undo the 1 ish bolts on top. there is one bolt underneath the egr valve which is a bugger to get to. you can remove the egr valve or if your hands are small enough you can get in there with a 10mm spanner. takes a bit of wiggling to get the cover off