The noise is probably from the air conditioner units which is why the person is unsure how often they operate. They can be quite loud and irritating.
The neighbor's house next door also has air conditioning. Why would the hut have to be louder than that?
They don't necessarily have to be louder but the two AC units shown in the photo appear to be of the 'louder than a car idling next door' variety. At full bore on a hot summer day you probably wouldn't want to try having a conversation within 10-15ft of them. Hard to say without knowing the details of the sizing or any aftermarket soundproofing they may not be visible in the photo. The neighbors may have a legitimate reason to be upset.
There's probably more heat to be removed from what's essentially a small datacentre than from a residential building.
I think in the case of one of the huts they have already installed, it was taking up a quarter of the park total land.
https://drive.google.com/file/d/0B4yHnbgE5ocJbUJpWFJGYUVVajg...
Is this really NIMBYism? People honestly can't stand a small utility building? It's strikes me as slightly odd that the person is sure that the huts make a lot of noise, but unsure if they operate 24/7 - could this be a way of phrasing that the information comes from a third party?