Worrying gearbox whirring!

Hi all, was out driving in my defender 90 200tdi the other day when a rattling noise started. i thought it was grit in the brakes so stripped them and cleaned them. the noise persisted. while offroading yesterday, the rattling stopped, but a very loads whirring noise started. took all the fllor panels out and drove it to establish where noise was coming from and decided it was the transfer/gearbox. The noise starts in 2nd gear at road speed and gets loader as gears and revs increase. If i depress the clutch and free wheel, the noise carries on and gets quieter as the truck slows.
Don't know if it's related, but the transfer box lever has been very reluctant to engage diff lock in all positions. only managed to get it in the other day by working away at it. could it be that the diff lock hasn't disengaged properly at some point and it's now buggered??

Any help would be really gratefully accepted.

thanks.

drain some oil out of gearbox and t/box see which one got metal fillings in oil