the whole usa usa usa usa thing generally is supposed to be rabbits sounds, at least to the japanese ear. I don't recall Reisen ever using it(well she is an alien rabbit) but Tewi, an earth born rabbit diffenetly uses it.
KOISHI: Sis, why won't the chest-part stay uuuup?!
RabbitNya... Nyaaaa?Just once, Rabbit-san... I want to pet you...Bun...bunny! Bun... Um... uuuh...
(I think Koishi's trying to make rabbit sounds.)Ribbon standing up.