I just don't remember the scientific term in English, it is that when you speak you actually produce two sounds at a time, it's produced because the chords are swollen at a point and not all over the muscle, so the air goes through two (or in some cases more) different "holes", we use an instrument called expectograph (I'm translating here) to determine if there's an only stream of voice or more because sometimes it's very hard to know, the instrument shows waves quite similar to those in window media and it's also used by doctors, the metallic sound is a distinguishible change in the colour of the voice, sometimes due to breathing problems, both things were quite easily observed in Bono voice during the Elevation tour and they have been improving ever since the beginning of the Vertigo tour, but his improvement became spectacular on the last leg and now I think his voice is quite healthy, but you know, I'm not a doctor, I hope I'm right.