1999 Pontiac Grand Am lifter/rocker arm

While running the car to purge the air out of the cooling system after adding coolant the car started making a loud ticking noice from the front bank of the engine. I am assuming it is a lifter/rocker arm. The car was overheating prior to and after changing the thermostat. Which is how I believe air was introduced into the cooling system and why I need to purge. How hard is it to idenitfy the rocker/or lifter and to repair myself. What would cause the rocker/lifter to start doing this when it was fine before the purge of cooling system. Checked oil and no water in oil, it is clean and full. Or is there another fix before besides replacing. When increasing the engine speed while in park it quiets down.