Hello Scudie!
We tried to reproduce this noise, but we did not succeed.
We will try to change the power supply (American) to try to see if it is not that which will generate this noise.
-Otherwise we will try to filter this problem by adding capacitors.
-We will also try to change the amplifier with a more stable faultless power supply and possibly put a shield.
We will be back soon with more answers once we have prototyped a new version !