[71SWBChevy]

Yeah that is the biggest problem the sound one man calls a ping another calls a knock will another calls a sound similar to a small mammal who is slowly being eaten by a rather large mammal being attacked by a mean man wearing a head dress and sporting a hatchet and bow and arrow.

also with it just happening on occasion it makes it harder to diagnois. I would recommend starting with the basics, you need fuel, air, and spark to fire a cylinder check and make sure you are getting all of these reliably. Also when the plugs were installed you did gap them to the correct specs right?