90 300tdi does same thing, also did my range rover . its annoying to say least . on my present 90 i found that the backash in the front diff is fairly high , almost 60 degs of rotation before it moves anything .
my other N/A diesel 90 is not so bad but less backlash and of course much less power to it .
on the range rover i found the noise was either coming when i went round corners or when i slowed down and braked , and also when stopping and the chassis "rolled back " upon stopping .
the clunking was due to one of the bolts holding the front panhard rod to the axle . mainly it seemed that the bolt was slightly loose and the hole in the axle bracket had enlarged slightly .
with the bolt being not quite tight enough to stop it moving , youd get it slip everytime the vehicle rolled around whilst driving . it is just a clunk that feels as if it comes from under youre feet .
i couldnt seem to get it to make the noise whilst i jacked it up and checked manually by hand , but it would do it when driving .
you can try tightening the 2 bolts up on each side of axle , but i think mine was just drivers side that needed doing .
i welded a washer on the axle bracket because the hole was enlarged too much on my brackets and the bolt still moved even with it tightened up more .
if you get clunking whilst changing gear , ie using clutch , then thats the slack in the front diff , allowing prop too much slack , mine does this .