That sounds to me like the slipper clutch is letting loose. A loose differential generally won't slip very long, just long enough to create that harsh sound. Most of the time you won't notice it until you try to listen for it. Add some tension to the slipper. It should go away and you should notice quicker off the line acceleration. Actually, you can take the clutch out, lightly sand both of those yellow octagon shaped pads to remove any shine which is caused when it slips. Reassemble and readjust as necessary.