Going to revive this thread as I want to provide closure to people in the future. I think @Sleepless was closest to the answer - when you have an aggressive turning angle, the brake pads are just loose and clicking around. If you tap on brakes just a little, it will go away. This is because when we turn the wheel, due to gravity and the position of the brake caliper relative to the ground, the pads are going to move around. This is of course due to caster. Make sense?