Hi,
I was driving my Disco 300tdi with a heavy trailer on the back and it started making a loud clacking noise but only when the clutch is released and in gear. It doesn't however make the noise in 4th gear, which I found strange The noise got quicker as my speed increased.

I was driving around a field in low diff lock about an hour before the noise started. And must admit I was still moving slowly when changing into high gear when I got back onto the road, and it made a loud crunch

Is this gearbox or transfer box problems?

Thank you in advance.