Sounds exactly like my problem. First I replaced the front pads and rotors because they were almost due. Didn't work. I had nearly new rotors and pads (aftermarket)on the rear. Thought I had it licked, but NO. I finally gave in and went to the dealer. Got the service guy to ride with me. The head service guy that really knew Volvos. It took awhile to make it happen with him in the car but finally it did it, just like riding over a rippled road, heavy vibration. He immediately said it is the rear and I should go with OEM pads. I then replaced the rear pads with OEM, even though they were still at 90% and only a couple of months old. Didn't want to replace the rotors since they were so new, & no warpage. I sanded the rotors. The OEM pads seemed to do it for awhile but then it raised its ugly head again. So I replace with rotors with OEM and am a happy camper again. This process took 4 months!

Bottom line: put on Volvo pads, Volvo rotor, and Volvo cut-away type rear shims. The service guy said that is the only sure fix and I believe him.