Yet another problem
 

Apparently I now need to fix a loud clicking noise that appears to be a valve problem. The clicking noise can be heard at low idle but not when driving and engine is reved up. It appears to be coming from under the passanger side valve cover. Will I need any special tools to maybe tighten the rocker arm nut in a 99 Disco 1? (If that's what needs to be done.)

You may find you have a broken rocker arm... rather than a loose one. No special tools that I recall though.