Vibration from almost underneath gearshift especially when turning a corner from start, Vibration goes away after vehicle gets in a straight line

How to diagnose clogged pcv breather

with 84,000 miles on the car and the car running normally and well suddenly there appeared a loud tapping/knocking from the upper engine head area around #5 cylinder. Spark plug ok, coil changed to check performance. Very rough idling and very sub normal power. Tapping/knocking briefly disappears un...