In order to build a microphone I used a website as a reference point:
https://felixblume.com/hydrophone/
Recordings of tap water:
Recordings of rain
Conclusions:
The hum occurred during recording, which is the result of bad connection of the piezo wires to the mono cable. I also experienced instability when recording a signal. Therefore, the DIY piezzo underwater microphone will not be used for further exploration.
