I there,
I think I solved my first « undesirable noise » issue ! I can't exactly explain this noise though...
I took the whole thing out of the enclosure (Bela, connectors, pots, buttons etc.)
The enclosure beeing conductive (the saucepan), there had a connexion between several masses due to uninsulated components (power socket, jack out, sensor connectors etc.).
Plus, I think I had a very bad microphone connection option : cold point to groung which meant a important loss of db amplified through the PGA. I switch to a proper external pre-amplifier.
I as mentioned I can't really say where this weird noise was coming out from but I'm so happy to have got rid of it since it means the almost end of a three years project.
Thank you again the Bela community !