Why do baby chicks make a short "whistling" sound when I touch them?

[Kr1spy_dreamz]

Mostly when I touch them on the back, they stop doing those sounds when i keep touching their backs for a while. My 3 months olds do that kind of chirping but the mature hens don't. Its probably some sort of thing prey animals do but it doesn't hurt to ask right?

 

[sourland]

Can you post a video ?

 

[Kr1spy_dreamz]

Can you post a video ?

You start to hear it at like 14 or 13 seconds or so.

 

[aart]

Typical vocalization for a bird that age.
Their voices will be changing soon.