BREAKING NEWS !
I discovered a unexpected way to get around the "whistling" noise problem - temporary solution but still a solution !
Someone asked me if by chance I tested with one of those single stomp boxes of the close past. Well, I've got two of them forgotten in a drawer since long time, a Boss Chorus and a Guyatone Phaser. I tested and the Phaser stops the "whistle" even in bypass! (Not the Chorus though). So that means that there IS a way to filter the godforbidden thing. For the moment I still wait for the tech person from the Telecommunication company comes to detect the source and to see how it will be sorted out. After testing with a battery powered amp and still having the noise it's now sure that it is an EM interference from some source. But at least I can continue my work till then. Huffffff..........
Thank you guys all for your valuable feedback.