It was supposed to emulate an Indian war cry, here's a vid of the last rebel vets doing it in the 1930s
https://www.youtube.com/watch?v=s6jSqt39vFM

There was a recording made of a group of Confederate veterans doing the Rebel Yell. It sounded like "yip yip YIP YIP".

Maybe something like this https://youtu.be/0GSI4jOQVN8?t=52

Kinda like the Xena war cry