Had exactly this on my old setup, its the mintex pads m8, 99% sure of it. I kept taking mine out every 2 weeks or so, rubbing the surface down and slicking the backs back up as they kept glazing over and causing the high pitched squeal (it used to get really bad) I know others have mentioned this with the mintex pads, I think its the material they are made of, it went when I put my crappy old ebc pads back in