91 940GL B230F & AW71L

Recently I was told by a mechanic that my car's centre universal joint was worn. I've just been under the car this afternoon and I don't think he was entirely correct.
If I hold the rear half of the drive shaft still and try to rotate the front section there is absolutely no movement at all. However, if I hold the *front* half of the shaft still and try to rotate the rear half then I can hear and feel a knock which seems to be coming from the vicinity of the centre bearing. The middle universal joint is not moving at all. Am I correct in assuming the centre bearing is worn? If so, why would it knock when twisted one way and not the other. Is it possibly worn on one side only???
There is also a faint whining sound which I had thought was coming from the transmission as I can only hear it in first gear. Perhaps when the gear changes, the bearing moves and quietens down???

Any thoughts greatly appreciated
Regards
Brad