350 ticking noise but only at idle. What would it be?

[SnS C10]

I posted this in the suburban forum but thought it may be more of a question for here

Hey guys long time member. My name is Shaun. I recently traded a toyota corona I had for awhile for this 1986 suburban. its a 2500, 4x4. engine is a crate 350 from Goodwrench with im told 40,000 miles on it and same for the transmission which im told is a th400 although i have not verified this. The engine runs beatifully and sounds great with its dual flowmaster mufflers. its all stock as it needs to pass california smog which it has.

My issue at this point is there is a ticking/knock noise. While in Park it will make the noise but with the slightest raise in rpms, say 100rpm, the noise goes away. There is no noise while driving or revving. Seems to only be right around the park rpm. Kind of sounds like the flexplate tapping against the cover but what do you guys think? I didnt think it sounded like a catastrophic problem and if it were i believe i would hear it more often at different rpm ranges. Thoughts? heres a link to a video

https://www.youtube.com/watch?v=QQvgLrDh0fA