Well, I'm going to eat my words. I stopped into the local dealer and had the mechanic ride with me. As soon as it made "the noise" he said: Do you have the shim kit? (Meaning the updated half-shims). Needless to say, when we got back to the service department, I bought the shim kit. I'm going to replace the rotors while I'm at it. I'll reply again if this doesn't fix the problem, but you can all assume it did if I do not report back.

I really need to pay more attention to volvospeed.com's Bay 13 section...there's a mention of the shim update in the "rear brakes" section.