It's just a random sound effect she makes to express surprise and curiosity.
Manga/anime girls often have signature strange noises they make in certain circumstances. Mao-chan from Rikujō Bōeitai used to make a peculiar AH-OOH-EH-OOH sound when she was distressed. And Maako from Burakku Gakkou goes "Poyo!" like all the fricking time, with variants according to mood -- for example, when she's sad she says "Sagepoyo!" It's just cute noises, there's nothing deep to it.
