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).
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?