The VonMuggenthaler recordings were captured in 2003 near Button Bay by audio analyst Elizabeth VonMuggenthaler and a team of fellow analysts. The recordings are of sounds coming from underneath the water that strongly resemble echolocation; specifically cetasian echolocation, that is most similar to that of the orca whale.
Of course, Lake Champlain being freshwater, nothing in the lake is supposed to echolocate. Although the sounds bear some resemblance to other, more common lake noises, the waveforms and pitches displayed in the spectogram set it apart from those species and leave its culprit unidentified.
The Mardis/Dranginis recording was captured on July 4th, 2017, at the Bodette film site, which is the southernmost mouth of New York's Ausable river.
The recording, although occasionally obstructed by other environmental sounds, bears a striking resemblance to the recordings captured in 2003, and also retains the same features that distinguish it from known species in the lake.
The VonMuggenthaler Spectogram
The Mardis/Dranginis Spectogram