Dunno about FG but this is normal on B series with LSD, would guess it's the same diff. Sometimes I can hear mine doing this too. The way you can tell if it's normal is if it only does it when the diff is warmed up. So first thing in the morning, go do some tight turns around a roundabout or corner. If you can't hear it, go for a long drive (at least half an hour) on the freeway, then test again. If you can hear the noise now, don't worry, it's normal. But if it makes noise when the diff is cold, something could be wrong.