Ok, so the raspiness of the idle is coming from the straight through "flowmaster" like mufflers.
A turbo type muffler will give a much different and quieter sound.
This is my 416 with the one size bigger 274-S solid cam, single plane intake ported/matched , open element air cleaner, full TTI exhaust headers/X-pipe/turbo muffler, ported Edelbrock heads.
So in the background of the exhaust you are hearing the mechanical tappets in the vid.