I pulled the rear calipers apart and found that the left rear caliper was full of gunk and the right one had some plugging of one of the orifices. I replaced all of the flex hoses in additon to rebuilding the calipers. The noise has dissappeared for now and I've been driving the car for most of the week. I also became a firm believer of changing the brake fluid as MB suggests. I purged the entire system and now have clean brake fluid flowing through the system. The old fluid was dark and looked a bit thick.
