Same thing happened on my 64 GMC 305 V6. Engine started knocking when the engine was idling. Turned out to be either a sticky lifter or a piece of carbon wedged in the valve. Does yours sound similar?

https://www.youtube.com/watch?v=HyhgUvL-2wU


1966 C10 292/tko600 http://67-72chevytrucks.com/vboard/showthread.php?t=596643
1964 C20 292/sm420