Ok, I got a little forward step here, I believe.
First of all I managed to identify the mains pick up source by observing changes in the input wave reported in my software oscilloscope
(I'm proud of that little piece of software
). I've replaced my voltage supply to fix this, I believe my transformer cables where acting as antenna (about 1m/3' wire lenght)
Now I have an almost flat input with no signal, but I still have some sort of reasonance/disturbance: my best guess this is caused by signal interferences due to the breadboard/wiring.
Does it sound?
Note: green wave is picked before amplification, blue wave after amplification.UPDATE
: I've added another attchment referring to a 33Hz sine wave: it seems clearer the interference going on