please help
 

I have an 1990 audi 90 quattro and i have a duff wheel bearing sound comming from underneath the car when it gets up to bout 40mph. ive been told it could be the prop shaft, so i went underneath it and i can move the prop back and forth a few ml. does anyone no if this is normal? does anyone no if topping up the diff oil etc might quiet it down a bit. any help or info would be really appreicaited. cheers