Torque bind again

My Legacy was just diagnosed with "torque bind". The service dept. fixed it and there is no more bind, but now there is a terrible "jackhammering" sound when I take my foot off the accelerator and they can't figure out what is causing the noise. Here is what my ticket says: "Test drove and diagnosed torque bind conditino. No noise heard. Replaced clutch pack set. Re-installed. Noise heard. Removed again and installed bearings. Slight noise heard. Removed 3rd time and installed shaft and bearings. Re-installed and drove. Very slight noise heard. No torque bind." The noise sounds like I'm driving by a construction site and there is a jackhammer off in the distance. It seems to only do it when my foot is off the accelerator and stops and soon as I press on the accelerator. Any clues?