creaking door sound

hey all,

So i live in central Ne and i was walking outside and i heard this creaking door i approached it i was able to figure out that it was coming from a pile of rocks next to one of our metal buildings. I got as close as i could before the noise subsided. I am an avid wild lifer and have a masters in biology. I have not heard that particular sound before. I think it could be a toad/frog but im not sure of the noise...any ideas???