What you're hearing
Brown noise puts most of its energy in the low end, so it sounds like a distant rumble or a waterfall heard from indoors, without the hiss of white noise. It's a favourite for focus because there's nothing in it to listen to.
It's made in code, so it has no seam and nothing in it changes. Add Light rain to one side, as in the Deep work starting point, or send it further away until you only notice it when it stops. The sleep timer fades it out after 15 to 90 minutes.