I also had been fighting this, up until this early evening. This had been bugging my wife for weeks, so this last weekend I got underneith the back end and started looking around. Being rubber what rubber is, it has a tendency to dry out. With that in mind i took a can of Invisilube, it sprays on like thin oil, penetrates then gels up like a grease, so it has a chance to get into hard to get into areas, like rubber bushings. After I let it set a few minutes to gel. I went for a little drive, and behold noise gone, and a couple other small noises were gone too. I had hosed down everything in the back end that looked like rubber or a bushing. Worked like a champ. Cheap too, the Invisilube came from my work truck. ;)