During spring and summer there are frogs croaking in pretty much any pond in Japan, so it's a bit hard to believe Sena has never heard them before.
But yeah, some species of frog can make some rather spooky sounds, so I think I kind of get it.
We used to live near an are, which could only be described as a swamp. All the frogs, toads, cicadas, crickets would get LOUD at night. It was like a symphony of creepy crawly things.