Sometimes the chicks do that. They stretch and make warbling looking motions with their throats but no sounds come out, at least none that we can hear, I guess.
As an experiment, you could try separating him from the group for 2 minutes. I've found that separated chicks often make chirping noises. The, "I'm lost!" chirps.
Alternately, they hunker down and stay really quiet. Another instinctive reaction to being separated.
I currently have quail that do both. Curious George will make a racket if separated from the flock. He always has. I predicted he was a boy from the start because he always seemed to be the chick in charge and looked out for everyone else. Houdini totally disappears and stays quiet and hides. She never makes a peep. She's every roo's favorite and has a bald head to prove it, so when she's hiding, I let her be. I figure she deserves the break.