I was introduced to the Black-bellied Whistling-duck (Dendrocygna autumnalis) on my first birding trip to S. Texas several years ago. While touring the King Ranch as a new birder, we encountered a pond within the immense property that was literally filled to the brim with these noisy waterfowl. We heard them before we saw them. There were so many, one could hardly see the surface of the water. In his book Essential Field Guide Companion, Peter Dunne describes them as " very vocal, but not necessarily loud. Their call is a squealing whistle followed by two (or more) high, sharp stuttered peepings." While they may not be very loud, when there are hundreds, their vocalizations fill the air.
Most recently, I've had the pleasure of observing the Black-bellied Whistling-ducks on my visits to the inner-city Brackenridge Park in San Antonio, Texas.
(click picture to enlarge)