Last time I heard random plastic noise inside it was my dog playing in the loungroom with a dead cockatoo, the sound was the wings rustling.

I've no idea where it came from, I've never seen a live cockatoo in the yard at any point. My guess is it suffered a random misadventure while flying overhead and dropped into the yard for her to find (this was not Mr Woof, this was a few years ago when Madam Woof was still with us.)