What could this odd noise be?

I have recently aquired a series 3 with a 200tdi. A few weeks ago when I started it, there was a terrible noise coming from under the gear tunnel sounding towards the front which I can best describe as a cement mixer full of dry gravel! I popped it into gear (it went in easily) to move off the drive tried to move forward and the noise remained so popped into reverse to pull back and the noise stopped.

A week ago the same thing happenned again...again on start up but this time not quite so loud. I left the engine ticking over and eventually the noise stopped. It happenned again yesterday so seems to be becomming more frequent. I checked the oil in the gearbox and thats fine and when running on the road all gears engage perfectly and no nasty noises.

Has anyone experienced this "gravel in a mixer" type noise or can point me in a direction for solving it?
Thanks
Dave

since it does it when stood must be in gearbox or clutch housing and going when putting in reverse would suggest reverse idler isnt sitting in right place and is catching 1st/2nd synchro hub

Thanks for the reply James. As the Land Rover is under a garage warranty they now have it back to investigate. I'll let you know the outcome....

A pebble in the gearbox housing.

As promised...here is the verdict. The garage think the problem was caused by a faulty ignition switch that was not releasing properly after turning the key and so keeping the starter motor permantly engaged. Seems to make sense...
Sorry for the delay in updating...only got it back from them today!