This is almost considered to be normal on Volvos around these years. Sometimes it is pad transfer from the brake pad into the rotor. Sometimes it is rotor thickness variation. Rotor warping is usually the least reason because they must be warped quite a bit to vibrate. You will notice that the rotor wears almost a fast as the pads, especially with OEM pads. Try Repco/PBR Deluxe pads if you don't drive too hard. The Deluxe's give you one good high speed stop and then you have to wait for them to cool down. They are easy on the rotor and the brake dust is much less. I use them on my 383ci Chevy powered 262GL that I drive every day for the last 17 years. Remember hard stop---must cool down.