My frog squawks like that if I mist him while he's sleeping. I guess he doesn't appreciate being woken up with a cold shower, haha!

It's happened a few times, back when I first got him, when I wasn't even near the enclosure. However, he's never made this noise when being touched or picked up. I can't tell you why they do it, but it's pretty safe to say that it's probably nothing to worry about. It's generally a distress call, yes, but sometimes they just do it for their own froggy reasons. It will likely calm down as it gets more used to its surroundings.