There is indeed a constant low and soft sort of twittering noise happy poultry make. Often they use a quieter version of the be-alert noise, which sounds a bit like a rattle or purr; usually roosters make that noise the most, and often it's used to draw the whole flock's non-panicked attention to a bird in the sky. Often it's ignored. It's not an alarm call, which sounds more like a dinosaur roar and gets all birds' attention. The rattle-purr thing is also used by hens on the nest and babies can make it before they've even finished hatching --- generally males are the most keen as they are born into the job of keeping an eye out and commenting on the world around them for the hen's benefit.
Also it's been my experience that dogs, goats, sheep and other animals that are smart enough (and some chooks definitely are) will learn to purr like cats, or some other noise, to show you their enjoyment of your presence. Often the reason they chose that particular noise is based on two facts; you respond to it favourably; and usually, it's a noise you make to them, in favourable circumstances. We often train without being aware of it. If they're imitating a purr it's often because they've seen you pat the cat for it.
For example, I tend to check on my animals at night a lot due to pythons and other reasons; when it's dark I talk softer to avoid disturbing them; also I use the softer voice when saying 'sweet nothings' as I pat them; this leads to dogs, chooks and even my sheep making a strange noise very similar to what I'm making. It's completely unnatural for them, a learned behaviour, and they use it when I pat them or when they're trying to solicit pats.
