On the last motor my dad built for my sisters car (360 with comp cams hydraulic cam, stock steel rockers, edelbrock heads) it would make a noise similar. It would do it on start up, sometimes it would go away in a few seconds, other times it would continue. Sometimes the noise would stop and after 30 secs start again. Finally determined it was the lifters. The block was a "newer" block (I think in the 80s, but can't remember for sure so not sure if that had anything to do with it or not) but anyways the center part of the lifter that is smaller in diameter was taller/bigger than other lifters we used in the past. Ended up buying a new set of lifters that had the smaller / normal size center part and the engine hasn't made the noise since.