I posted awhile ago about my wife's brakes making a horrible, loud grinding type noise, especially when hot and at relatively low speeds. It is like the whole car is shaking, grinding to a stop. The good news is that I am getting very experienced at changing pads and rotors. The bad news is I haven't been able fix the problem.

Here is what I have tried, in order:
1. replaced front non-OEM pads with Volvo pads and replaced front rotors. (for some reason I was focused on the front brakes, assuming that was the problem)
2. noise continued, but very intermittent.
3. took to dealer and drove it with mechanic. Fortunately I was able to get it to grind and he immediately said it was the rear brakes. He said to replace rear pads and rotors (dealer estimate nearly $400)
4. I replaced rear non-OEM pads with Volvo pads. Rotors were nearly new (Brembo)so I sanded those a little to rough them up slightly. Put on lots of Volvo silicon gunk on back of pads and guide pins.
5. no complaints from my wife until two weeks later. The noise is back.

Even though the rear rotors looked fine, I guess I will replace them. This time I will buy them from the dealer because they then come with new caliper bolts, although the old bolts looked fine.
What else can I do? Even though the guide pins looked fine, should I replace those? If I thought the $400 brake job from the dealer would fix the problem, I would gladly pay it.