I have the same problem on my 08 Mastiff. Ive done over 2000 miles since Ive owned it, then all of a sudden this strange noise. Sounds like an electrical short. As it was speed related ie. High pitch at speed, then as I slow down it dulls down. Sounded like it was coming from the left hand control. It started on a day that I had washed it aswell, so I figured Id gotten some water somewhere it shouldnt have gotten (which is 90% of the bike haha).
I know everyone has said that its from the rotors passing the pads.... But. Why does the sound start when the brake lever is only lightly touched (only enough to light the brake lights), but does not change the more pressure is applied to the brakes? Surely the extra pressure would change enough to change the noise? It sure does sound electrical.
I wonder if pulling the brakes off, cleaning the crap out of it and replacing the pads would fix it? Im OCD when it comes to rattles, buzzes and strange noises on my vehicles so this is going to bug the hell out of me.