Help with recreating this effect in Reaper

https://youtu.be/3P5gbHsEnwY

So I'm trying to recreate the echoing flanger like effect at the end of the Burning Man's sentences, that kind of echoing whine in the air. I already know how to create the deep, demonic tone, but after screwing around with the flanger, I'm at a loss. Any ideas?

Thanks.