My 300 tdi has developed an odd clattering noise which i am struggling to diagnose

Truck stationary
Apply quarter of a turn of the steering wheel
Resulting clattering noise which disspaears when bring wheel back to centre

Does it on both left and right hand side

Is not the usual whining or moaning you get on full lock

PAS reservoir full and nothing obvious

What do you think ? i have had various suggestions so far from PAS pump to steering box to CV joint

Thanks