Mystery mystery

field recording undetermined (sex) uncertain I am stumped by this nocturnal animal sound recorded along the Brazos River in Sugar Land TX. Lived here a long time and don't remember it in the past, but have been hearing it fairly regularly for last couple of months. Never heard during the day. Appears to be more than one. When I first heard it a couple of months ago, I thought possibly Least Grebe. There is a vernal oxbow pond nearby where I have had LEGR before. I've convinced myself it's not that. I went through all the local frog calls and no match? I'm usually able to figure stuff like this out. Beginning to wonder if there's a new invasive species, as sometimes I think I hear 2 or 3 at different distances. ID please?? Thanks!. . Habitat is Brazos River bottom deciduous forest behind suburban neighborhood.. Some intervals between call have been shortened a little.; playback used:no