I have conducted passive acoustic monitoring at a nature reserve (a wetland, near a city and two medium-sized roads). 
At night only, I have a strange source of sound that goes on and off every now and then, ranging between ~9-18 kHz (see spectrogram from 1 AM).

Edit: At night I recorded only from 01:00 - 02:00 because the night is not the main focus of the acoustic monitoring. It is repeatedly occurring during this time window (aka not a singular event per night), every night (for weeks), in 2020 and 2021 (in both - autumn).


 [![enter image description here][1]][1]


Edit: I am now adding another spectrogram where "three such sources" are seen "together":
[![enter image description here][2]][2]

I do not know what the source is. Nature reserve managers are unaware of any water pump nearby, but I suspect this has to be the source of such a sound. 

Can I rule out any biotic sources assuming no animal here (birds, bats or insects) can produce such a broad range of frequencies?
Any ideas about what could it be?

Thanks!


  [1]: https://i.sstatic.net/i4E3o.jpg
  [2]: https://i.sstatic.net/lFTpX.png