could this just be an adjustment on the valve tappets? or something more deeper such as rocker arms, pistons? noticed it when i switched to fully synthetic oil.... or it could already been existing for quite sometime. just been more pronounced when i switched to fully synthetic. i only hear it though on idle...

one more thing.... might be related to my first problem. when i switch on the ignition key to the ON position without cranking the engine yet, i hear a buzzing sound. could it be the starter giving in anytime soon? already replaced fuel filter and battery.