The Rebel Yell’s Origins- 

Confederate veterans recreate Pickett’s Charge and the “rebel yell” during the 50th anniversary (Pennsylvania State Archives) and 1882 view showing ground Pettigrew’s and Trimble’s brigades walked (NPS)

One of the earliest accounts of use of the yell comes from the First Battle of Manassas (Bull Run) during then Brig. General Thomas Jonathan Stonewall Jackson’s assault at Henry House Hill. An order was given during a bayonet charge to “yell like furies”, which was instrumental in routing the Federal forces under General Irvin McDowell back to Washington D.C., "Then arose that do-or-die expression, that maniacal maelstrom of sound; that penetrating, rasping, shrieking, blood-curdling noise that could be heard for miles and whose volume reached the heavens-such an expression as never yet came from the throats of sane men, but from men whom the seething blast of an imaginary hell would not check while the sound lasted."

-Colonel Keller Anderson of Kentucky’s Orphan Brigade, "It paragons description, that yell! How it starts deep and ends high, how it rises into three increasing crescendos and breaks with a command of battle."

In Century Illustrated Magazine (1892), “At last it grew too dark to fight. Then away to our left and rear some of Bragg’s people set up ‘the rebel yell’. It was taken up successively and passed around to our front, along our right and in behind us again, until it seemed almost to have got to the point whence it started. It was the ugliest sound that any mortal ever heard — even a mortal exhausted and unnerved by two days of hard fighting, without sleep, without rest, without food and without hope…” - Narrative of then-Lieutenant Ambrose Bierce, 2nd Brigade, 2nd Division, XXI Corps, Army of the Cumberland, at the Battle of Chickamauga (Last Union defenses on Horseshoe Ridge, September 20, 1863),

William Howard Russell, war correspondent for The Times describes the rebel yell as follows: “..the Southern soldiers cannot cheer, and what passes muster for that jubilant sound is a shrill ringing scream with a touch of the Indian war-whoop in it.” 

http://www.mtv.com/artists/rebel-yell-00/biography/ and wiki