Discovery 2 Drive Problems

Hi to all. I am looking for help please with a problem on my late 2002 Disco 2, which is,whilst driving I am getting a thud which appears to be drive related, it happens at various times when driving also when pulling away, not every time just now and again but it is sometimes loud sometimes quite, I have had truck checked by Tonks 4x4 locally and he said all uj's and couplings etc were ok. Thought possibly it could be transfer box, any pointers or help welcomed.

Thanks
Bob

Forgot to say its a manual gearbox.

Bob