your rear rotors could be warped, but it could also be the pads or shims. I've heard that Volvo rotors shouldn't be turned, not sure why though. Anyway I have the same problem, but only when the brakes are warm and only every now and then which leads me to believe its not the rotors becuse it would be more consistant. From previous posts here and on Volvospeed.com, it's been discussed, and a few have suggested changing the pads and or shims to see if it goes away first, before trying new rotors. I've got new pads and shims on order, we'll see if it works next weekend!