Every night it sounded like there was a slow, gentle rain falling on our tent. But there was no rain, there were barely any clouds in the sky. I think the sound was that of frass (caterpillar droppings) falling from many unseen caterpillars in the Hackberry tree by our tent.
White-marked Tussock Moth - Orgyia leucostigma
Pale Beauty - Campaea perlata
Grapevine Skeletonizer - Harrisina americana
Banded Tussock Moth - Halysidota tessellaris
"Caterpillars of Eastern North America" by David L. Wagner