Noisy diff?

[rs phil]

my gearbox seems a bit noisy when going slow slightly feathering the throttle in all gears,was wondering when diff gets worn does it become noisy like metal on metal rattle or is this just normal,has 50,000 miles on it and some track use,i replaced oil recently with recomended 75 w90 silkolene pro s oil,and i think the noise wasnt there before maybe an additive was in there to reduce noise before hand,when driven hard on track or in traffic shift is perfect hot or cold and no whining,some say the noise is crown wheel and pinion clashing which is normal...? and when driving quick takes up slack and noise goes away...help