The Rebel Yell 'en masse' has an eerie sound not unlike a raucous pack of wild dogs. I would think that hearing that sound from hundreds of men running at you with fury in their eyes would be quite intimidating, if not outright scary.
The best recording of it I've heard is this one (
https://youtu.be/7mGAesLYlF8?t=160 - it should start at 2:40). It was made using a recorded yell that was then patterned to replicate the sound of one company making it - an entire regiment or brigade yelling it must have something to behold.