It's a bleeding issue, I had the same with my Discovery, re-fitting genuine landrover master Slave sorted it, as it was leaking letting in air. Ended up with a new bearing, Slave and master.
To add to my last the garage has told me my exhaust manifold is up the Creek which has been contributing to everything so that is being replaced on warranty also.
Hi
Got a 2003 discovery 2 td5 es manual, first one I have owned, based north Derbyshire. 114 K on the clock but blowing some black smoke when accelerating along with a sooty coating on the exhaust tip!