We have a Clark C300Y40 with an F163 Continental engine. After the engine has been running for about 5 minutes, a metal to metal sharp type of noise develops. The noise only happens when the engine is at an idle. Once it is accelerated even slighly above an idle it goes away. We removed the valve cover to so if a valve or lifter was stuck and they are fine. Could it be a timing, idler or hydraulic pump gear problem? Any ideas??