I have been dealing with something very similar - I am not certain if the frequencies are in the range of what you are experiencing but I have found a device called a "Bat Detector" which is a simple circuit board that uses a directional mic array to help locate bats in dark environments. It plugs into a pair of headphones and has a frequency control knob so you can try to dial in the sound you are looking for. I don't live in an area as dense as yours but I would try mounting the detector to a tripod with some kind of sighting scope to be able to aim it with some precision.