Yesterday I discovered that the left front braking disc is sligthly smaller (in diameter) than what is is supposed to be. It's about 240mm instead of the regular 258mm which are on all european focus models. It seems to be a disc from a Ford Ka, which is 240 mm but other than the diameter is pretty identical to the disc of a focus.
This caused an uneven wearing of the break pads, with the top edges of the pads being virtually new as they slide over the disc.
Thus, I polished the pads to make sure this wasn't causing the noise I'm still experiencing. Again, this operation was unsuccessful. I'm kinda running out of option here...