If the characters in this universe aren't familiar with thunder or waterfalls, I guess I would question how they'd be familiar with the sound of logs rolling on a floor. Likewise rocks rolling down a hillside. It sounds like an urban setting, albeit post-apocalyptic. I guess they could have heard those things in their world, but I'd wonder why no thunder or rushing water.
Might they be familiar with the sound of a collapsing building? Or the rumble of a concrete pillar before it crumbles under its own weight? A tunnel caving in, an old subway tunnel for example, and the lingering roar echoing afterward?
Are there still motor-driven vehicles? Could they be familiar with the grumble of an old engine at idle? Or wildlife roaming their urban expanse, like a snuffling boar, or a bear?
Or, more intimate to the characters, an amplified version of someone's empty belly roaring for food?
I can't even imagine what it would be like to live someplace where I'd never heard an earthquake, the rolling sound of distant thunder, or a raging waterway, but if the characters don't have those in their experiences, maybe it's helpful to make a list of things they would be familiar with, and the sounds that list of things might make.
Perhaps one of those will be close enough to what you're looking for.
Sounds like an interesting setting. Good luck with it! :)