I suspect it is ABS modulation especially since your S70 is a '98. The fact that doing the brake work did not cure the problem supports the theory. When ABS is activated, it makes the same type of sounds you describe. Further, having the sustained noise occur when braking at a higher speed would happen as the brakes are being required to act under greater pressure for a longer period of time. The '98s have had their share of ABS problems...whether it is the sensors or the controller is anyone's guess. You may want to search the archives for more info.