A lot of genres get defined by their trappings rather than by the real skeleton going on underneath: it's like defining birds as "things with feathers" and then freaking out over dinosaurs with feathers. Sure the feathers--or ray guns--serve some purpose, but you can't entirely divorce that surface trope from the purpose. So all those cyberpunk stories were people wear leather and chrome for the simple fact of wearing leather and chrome--that's like gluing feathers to your dog. It's kind of pointless and kind of gross.
So: noir. Dark streets. Trenchcoats. PIs with shadowy pasts and hearts of gold. Sure, absolutely, but underneath those feathers, noir is about an uncertain and largely untrustworthy world. The conspiracy goes all the way to the top. The police are in on it. The love of your life will betray you. The noir world is one where the ground under you is uncertain, metaphorically.
So let's make that metaphor real, yay! (I love making metaphors real. If someone says, "Her world exploded," I want to see chunks of world flying off into space.) We don't see a lot of Hollow Earth stories these days, stories that aren't just "Let's go underground," but "Let's go underground and find a whole civilization there." In the past, we had Edward Bulwer-Lytton had The Coming Race in 1871; and Edgar Rice Burroughs had At the Earth's Core in 1914; and most famously(?) Richard Sharpe Shaver had the Shaver Mystery starting in 1943, which hovered uncomfortably between fiction and non-fiction, since he claimed the evil underground civilization was real. (Also, you could reach this world through several secret elevators, though considering the underground "dero" liked to rape and murder people, I'm not sure why you would want to go there.) But that's all old; it's well worth revisiting this subgenre of Hollow Earth stories (or "subterranean fiction").
Ergo: hollow world noir is a world where the ground under your feet is uncertain--because under the ground are a group of people who have a hidden agenda. (Plus: you get the fun of having someone say "This goes all the way to the top!" and then having to go all the way to the bottom.) Now there are a lot of ways to approach this: perhaps a noir LA detective discovers a murder that eventually leads him to the underground conspiracy. Or some people in your town are collaborating with the Hollow Earth gangsters. Or a man without options starts working for a mysterious man who just might not be from the Surface World.
Why aren't you writing... Hollow Earth noir?