I was raised in the Great Smoky Mountains where I was told that ‘yell’n from ridge to ridge or holler to holler was a form of communication both to let folks know you were in the area and to keep from getting shot. When the Northern Aggression came and the Mountain boys signed up to fight, hollering at the top of one’s voice just came naturally when they charged a Union line or position.
The trill was the part I didn’t know for sure.
You’re right- it goes back to the Scottish clans callin up for help.