Doesn't sound like much of an alarm, more like "hmmm, what's that sound".
...or could just be the trilling sounds they make as they settle for the night,
I call it' 'bedtime stories'.
Tell us more about your flock and coop/run.
-your flock size(numbers, ages, genders),
-your coop(size in feet by feet with pics),
...and how long have you had your birds?
Oh, and.....
Welcome to BYC! @Rcpopart
Where in this world are you located?
Climate, and time of year, is almost always a factor.
Please add your general geographical location to your profile.
It's easy to do, (laptop version shown), then it's always there!