metallic grinding sound when travelling :(

ok guys time to tell me the baddddd news!
landrover 300tdi defender.
metallic sounding grinding when travelling, not when clutch is pressed, not specific to road speed or gear. not been offroading, at most a few bumpy cun'try roads and speed bumps so surely not a bent axle.
defo sounds like its coming from underneath the middle seat
any advice ideas, costs, time i should expect it off the road I will not be fixing it myself.

Worn gearbox teeth, bearings etc... Is there oil in the transmission?

Could be as simple as a worn u-joint get under it and check along both drive shafts for any play. This is more than lightly whats wrong and its a small job you can do yourself and the ujs are not expensive. Good luck fingers that crossed Im right :

My Disco sounded exactly as you describe when it got a stone caught between the front disc mudguard/shroud (whatever it's called) .. don't forget, the transmission is a series of hollow tubes and sound travels easily .. Could also be a worn/dislodged disc pad ...

When you push the clutch it goes did you say?

still coulld be a uj when the clutch is depressed the load is off the uj Jai

My guess is low oil in one of the gearboxes or rear diff. It's certainly what I would check first. While you're under there check the UJs out.

wheres bloody ratty?