What does this noise mean?

[MaritimeFlock]

One of our rooster makes this noise sometimes. What does it mean? This time when he did it the other chickens looked on high alert. But normally they go about the activities when he makes this noise.

 

[Cloverr39]

I'm also interested. My silkie rooster does the exact same noise. I was debating weather to ask on BYC out of curiosity. I haven't been able to catch it on camera yet.

 

[HollowOfWisps]

Usually it's because they noticed something different, they are giving a warning or they are irritated.