Worrying Noise

Hi all
My 94 Defender 110 has developed a disturbing problem sometimes when I'm pulling of it seems to slip outta drive but not out of gear somehow and makes a terrible sound like grinding its intensity is matched by how much throttle it's given. It stops when I press the clutch down and take it out of gear.
I'm a landy owner for two years and love it to bits but I'm not too knowledgeable on the mechanical side so any advice would help and no selling it is not an option.
Thanks

Are gear changes becoming more difficult? With the engine off can you select all the gears easily? Sounds like it could be air in the clutch which is a simple fix.

Thanks for speedy reply, the gears all go in easily enough with engine off.
It's like it reaches the biting point then something pops out and makes a grinding noise.
Thanks