Betcha they're doing their job, alarming you to some danger. Watch them carefully, they usually point at the cat, or hawk, or rat they're loudly alarming about. Then if they suddenly go quiet, its a hawk, and they'll keep one eye pointing RIGHT AT IT! Hawks and owls blend in with the scenery very well, but if you follow your chickens' eye, you'll see it.
If you're in a city, you may never have noticed your predators before. Now you have chickens, and you'll have fox, coyote, hawk and other varmints. In our 'hood, there's even people who tresspass. Never noticed until the chickens told us all about it.
Now, the kids can ID the Coopers' hawk vs the SharpShinned, Red Tailed, Harrier and owl. We have a Screech owl pair and a Barred Owl, which is huge. Luckily, the chickens attracted the barred owl, which dines on the skunks. (But the skunks were around looooong before us or our chickens.)
I wish you could post a video of your chickens noise-making parties. I just bet its the alarm noise.