The tonal sound below 1 kHz that varies in intensity could easily be traffic noise from nearby roads. (varying intensity an irregular occurrence), The broadband noise from 9 to 18 kHz in contrary has steady spectral intensity and ends abruptly. This is typical for technical noise at constant distance with a binary (on-off) characteristic, the reason of which can only speculated about.

Here is my attempt:

As noise occurs only at night, when temperature goes down and therefore humidity goes up, it could easily be electric discharges of a close-by power line.

Maybe such discharges could also occur inside the recorder.

If you consider this a sound signal and carry out co-variate analysis (temperature, humidity, duration, etc), maybe you could pin-down the source

The tonal sound below 1 kHz that varies in intensity could easily be traffic noise from nearby roads. (varying intensity an irregular occurrence), The broadband noise from 9 to 18 kHz on the contrary has steady spectral intensity and ends abruptly. This is typical for technical noise at a constant distance with a binary (on-off) characteristic, the reason for which can only speculated about.

Here is my attempt:

As noise occurs only at night, when the temperature goes down and therefore humidity goes up, it could easily be electric discharges of a close-by power line.

Maybe such discharges could also occur inside the recorder.

If you consider this a sound signal and carry out co-variate analysis (temperature, humidity, duration, etc), maybe you could pin-down the source