I would always recommend to set the diff first and THAN the slipper because the diff setting affects the slip too.
What is the sound like?
a BSSSS or a BRRR
BSSS is slipper BRRR is diff
It is really hard to see the slipper slip, but if it slippes it gets very hot after one or two short slips. So you can try with your finger, but be careful.
Just clean the diff plates, dont sand them because they soulnd not sand down the diff pads
IU would leave tham as they are.