I did a manual Archlinux installation in order to examine if this noise is still there in a minimal headless installation.
I installed it successfully and the noise (like the noise that old vinyls do in turntables in a digitial version ) is still there with both samba and nfs shares.
As it seems my installation suffers from the shared ethernet/usb port and i can't do anything about it as it's not a software bug.
I thing it's time to move on to my next embedded hardware (cubox i-4 pro maybe?) as i've lost countless hours trying to figure out what's happening.
FYI in pure arch installation i had no problem with audio lavels changing. It always stayed to 100%.