Rumbling brakes? Any vibration associated with the noise while braking? If yes, then your brake rotors are likely warped. This could be caused by many things including improperly torqued lug nuts. If the rotors are warped, the only real option is to get new ones--Volvo advises against turning used rotors. Drilled/slotted rotors will only help to fix your problem because they are new, unwarped rotors. They do not offer any advantage to OEM, non-drilled/slotted, brake rotors in terms of resistance to warping (in fact, some say they are more prone to warping).