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.