I'm trying to figure this out...but I think I've got a slight problem. Everyonce in a while, almost fully repetable, I've found that my 03 wrx (74k miles) has a tendency to "pop" (its definately a mechanical sound) when I shift into 2nd and 3rd gears. I haven't noticed it in 1st or 4th and 5th gears. The pop is very clearly in the rear diff. I would have normally expected it in the front/mid areas from clutch wear. In analysis, I'm leaning towards gear lash whip in the rear diff from clutch wear (i.e. still a clutch issue). Has anybody else experienced this?