The synopsis says there are other girls around him with lewd quirks who'll show up, so I guess Kawashina is the first outside of Luna. Wonder what that "quirk" is.
I have a feeling the reason for Luna's moaning isn't getting explained for a long time. Partly also because he also refuses/incapable of telling her honestly that it was her nightly moaning's fault that he can't sleep... I'm just surprised she seems so completely oblivious to the possibility it was her very loud moaning that was the root cause. Really? This is a situation that proper communication could easily solve, but of course we're never getting that in a coombait teasing story like this. I'd be surprised if they explain it soon enough.
My theory that she's an AVtuber streaming on FC2 or something or doing erotic VA recording for stuff that gets sold on DLsite still stands. If true, she likely wears a headset while doing streaming or recording, so she's less aware of the actual noise she's generating around her.
Actually playing it straight and revealing she's got a partner she's doing it with on the regular I feel will drive the target audience of this ecchi manga away, so that's unlikely the case since that's a bad business decision. Playing with herself is a bit more likely than having a 3rd person over, but it doesn't explain her seeming lack of awareness about it.