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

I do not know what is the source. 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 source assuming no animal here (birds, bats or insects) can produce such a broad range of frequencies? Any ideas about what could it be?

Thanks