Both Luna and Ayaka's ears twitched hearing the noise themselves and were focused on trying to decide on where it was coming from. Sure it sounded as if the volcano was about to erupt, but the two of them knew not to just let it go as that until they were absolutely sure. 

Luna turned into a white cat and sat on the ground listening intently. She figured her cat like senses would be better in cat form.

Ayaka started using her wind powers to listen thinking that would help. "My wind isn't picking up a sound like what I heard......"
"Maybe it didn't come from above ground." Luna suggested.