Gen or Luffy/Robin if you squint - Luffy, Usopp, Chopper post Enies Lobby - PG
The sound drifts through the new Thousand Sunny, thin and high, oddly bright for its slow, mournful edge. Usopp is just drifting off to sleep when he hears it, and with all the restfulness and peace carried in the sound it shouldn't wake him up, and of course he knows it can only be one person, but... he wants to know why, and for some reason, he feels himself pressed to investigate.
He runs into Chopper at the door, and they shush each other quiet and tiptoe with an air of conspiracy, the bold explorers, boldly barefoot in nightgowns, following the trail of the mysterious sound. It takes them closer and closer to the girl's cabin with every step.
Nami is on watch, so only Robin should be there -- which means that it is safe, more or less, since a peek may only get them a dismissal by businesslike hands rather than the sort of extreme violence they'd be courting otherwise.
Usopp peeks in, fighting Chopper for the gap in the door (easily, because the other doesn't think to slip into heavy point). And he sees.
A smile creeps over his face and his chest constricts and he sniffs... but he's not crying, dammit, because he's a Manly Warrior of the Seas! so he sets his quivering jaw and lets Chopper push in front, who shortly gives a squeak, bursts into tears and starts hugging his leg.
A moment later, the sound falls quiet, the door slips open and they're discovered.
...But Luffy, wide eyed and blinking in his own overlarge nightshirt, just presses his finger to his lips and whispers, "She's asleep."
They scuttle away, and safe on the deck, outlined silvery in the moonlight like three small ghosts, Luffy adds, "Makino always used to sing to me when I had bad dreams. It's good. It makes it better... Robin told me she never had anyone to do that."