Noise source FOUND!
My neighbors are the bad guys... well they are really nice people, but still they are the bad guys in this story

My neighbor came to ask for sugar and also invited me for dinner. Of course I said yeah!

After lots of discussion and so, I noticed that they had an air conditioner and asked a little bit about it, etc... My neighbor told me he had a fuse exploding just a few days ago and that the air conditioner automatically goes to power saving mode in midnight. He also told me that his subwoofer did the same as my subwoofer did.
Problem solved and my neighbor said the ventilation in their apartment is good enough so they will sell the air conditioner second hand = EVEN BETTER!
Don't know what's the deal about the air conditioner making that interfering signal, but it seems to affect all subwoofers connected and powered to an outlet. Also due to the fuse exploding a few days ago, my neighbor had to change it and he said that it happened just before midnight, perhaps 2 minutes or so and the air conditioner delayed before going to power saving mode because it needs to operate normally for around 5 minutes before doing any configuration when it is powered on for the first time or any other time after you turn it off.
Well, we didn't test the issue in that I would have asked my neighbor to shut the air conditioner down, but all pieces form a nice jigsaw now so I simply can't be wrong. Pretty funny, eh?
When doing further investigation, the air conditioner is positioned exactly on the same mains power rail as where my room is and where my PC is connected, etc...
Thanks for the help, but it was really fortunate for me to solve it by offering sugar to the neighbor
Be nice to your neighbors if they're nice to you
