Characters humming parts of their character songs (this along with some occasional chatter happens if you idle long enough while customizing their looks)
Note: some hum their songs longer than others
Airi
Amane
Asuka
Haruka
Mari
Mio
Nao
Nonono
Reika
Riho
Rui
Setsu
Bonus: Here is some of Set-chan's idle chatter