I pulled the drain plug from the oil pan and it is magnetic. There was not any trace of metal shavings or metal debris.
I ran the motor and listened with the stethoscope near the valve covers and intake manifold. The sound seemed to be coming from the valve train for cylinder #4.
Next I pulled the valve cover and inspected preload (pushed and twisted the rockers) and found that the lifter for the intake valve for cylinder #4 was mushy. I have seen this problem before and in that episode two of the lifters had debris (oil filter element?) in the intake port that prevented the lifters from pumping up. I ended up replacing the full set of roller lifters. My next step is to pull in the intake and fix or replace the problematic lifter.