rear axel knocking sound

hi all you helpful people, we have a 1995 300 tdi manual. when driving it today it started with a slight vibration and a knocking sound from rear offside wheel. It seemed to only be there when you let off the gas. Anyway on our way home it was there all the time when driving and has got worse. When i move the offside rear wheel forwards and backwards, there seems to be alittle play, and a clunking to go with it. Any help would be really good, i'm sure its a common thing but as this is ourfirst disco i haven't a clue. I should also add is that the klunging seems to match the speed of the wheel. It almost sounds like driving on a flat tyre which i'm not have checked the pressure
many thanks in advance
alex

Wheel bearings and/or A-frame bushes.

ok i did forget to add that the knocking is consitant with the speed of the wheel.

hi all have been under the car today and have checked the a frame bushes and are all ok, and the wheel bearing no play in it at all. When turning the rear prop left and right the clunking seems to be coming from the either the diff or offside wheel. the clunking is constant with wheel rotation, weither it be underload or free wheeling. checked the uj and all seem smooth with no play. Looking at the props they seem fine ,look fairly new and have plenty of grease. I'm thinking either half shaft or diff itself,could i be wrong as i'm only guessing here. Also what sort of sound would a worn dougnut make, need help as i'm going crazy, found loads of different things for what it could be