I don't have a roo, but my girls always make a trill-like whirring high-pitched noise. Whoever sees it first does it and they all duck and cover or freeze, depending on where they are, and make the noise very quietly amongst themselves. It's clearly an alarm.
Today one of my girls comes barreling across the yard a gazillion miles an hour clucking like someone was skinning her alive, and I looked up to see a bald eagle curving overhead- we aren't far from the river that runs through town and we always have them this time of year. It scared her to death! Funny thing is, when she got under the playhouse, she started ba-gawking as if she'd just laid the largest egg, EVER! The other girls calmly ducked under a shrub and whirred.
**Edited because I'm a grammar nazi.