In July 2006, I heard these vocalizations at night in my neighborhood in mature hardwood forest in Orange Co., NC. It was a vocalization that I was unfamiliar with, and I solicited the aid of Carolinabirders in identifying them. These recordings are faint; be sure to adjust your volume settings to their highest values -- loud enough for the katydids to be unpleasant.
My birds only began calling when darkness fell, but some observers reported having heard a similar call by daylight, and seeing the birds. Based on their responses, and on a similar recording on the Stokes Field Guide to Bird Songs, Eastern Region, and on a brief glimpse of one of the birds as I flushed it on the 4th night of observation, I am identifying these calls as those of juvenile Barred Owls. Thanks to the many who responded to my question.
Later, a video posted at http://www.devilducky.com/media/52983/ confirmed the identification positively.