The engine started making a clacking sound from.

The engine started making a clacking sound from the valve cover area. It started after leaving the highway at 70 mph and started after slowing to 50 mph. The car will stall after a it's revved and throttle is let off. It sounds like a collapsed lifter. I ran engine flush and put in new oil and filter. I plan to pull off the valve covers and check the rocker arms for looseness.
Any ideas what would cause this, car ran great before this?