What you saw was a predator drill. When a chicken notices anything the least bit out of the ordinary, usually something our eyes and ears do not notice, they issue a subtle warning, and all the flock freezes to assess the possible dangers. Sometimes, the "purring" will progress to a low ominous clucking signally an escalation of the perceived threat, usually a land predator such as a bear, fox, or your neighbor's dog. That in turn can progress to an individual getting a physical sighting of the threat steadily moving in their direction and a five-note phrase is then uttered which tells the others that a possible threat is approaching.
All those chicken sounds are a complex language that utilizes syntax which puts sounds together to communicate information just as humans string words together to communicate to one another. Baby chicks fresh out of the egg already understand and speak this language, and that was my first introduction to chicken speak when a tiny chick I was holding uttered the exact same phrase when it saw my cat come into the room as my adult chickens would.
This five-note phrase has two different versions. One is five notes with no emphasis on any note, and that means the threat is moving to the next level but the full danger isn't yet assessed. The next threat level is a five-note phrase with the accent or emphasis on the third note indicates an imminent threat.
Once that five-note threat warning has been issued, the chickens will be prepped to flee if the threat makes a move to attack. Then you get the explosion of panicked chickens squawking and screeching and everyone running for cover or hiding.
Probably way more information than you asked for.
