Quiet in the Ranks! (Not.)
The soldiers always yelled on the slightest provocation. Day or night, in camp or on the march, they exercised their lungs whenever anything gave them an excuse for doing so. If a favorite general came in sight he received a boisterous greeting; if a frightened "cotton tail" rabbit started up it was enough to set a whole division yelling. One of those mighty choruses would sweep in a tumultuous wave for miles through a great camp or along a marching column, when not one man in ten had any idea what he was yelling at or about. It was violently contagious, and one regiment or brigade yelled just because its neighbor did.
No great undertaking that required united physical effort was accomplished without the inevitable yell. The men yelled when the bugle sounded for a rest from the toilsome march, and when the head of column filed off the road betokening the end of the day's tramp; they yelled at the sutler, the commissary, the quartermaster and the paymaster; they yelled with equal ardor at the sight of a pig, a chicken or a woman - for there were times when a woman's face was not seen for weeks.
(from Corporal Si Klegg and his Pard by Wilbur Hinman)