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

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.