The thumping, popping, and clicking could very well be stuff falling, but the machine sound...that sounded like a ventilation fan (very unlikely seeing as that place has been abandoned so long, but that's all I can think of that it sounded like).
I didn't hear the "sobbing" so I can't say anything about that.
A little advice though; bring a light with some crazy candlepower if you want to get a really good video down there and to help you see more. And make sure you're friend doesn't actually bring a gun, you wouldn't want him to shoot a cop or the landowner
Cool video all the same; was the floor mostly concrete or was there a lot of metal flooring?