Is the noise there all the time or only after a t̶h̶r̶a̶s̶h̶i̶n̶g̶
'hard drive'?
Could it be a diesel knock due to a faulty injector, or is it more metallic?
If metallic it could be any number of things that would require investigation
Hydraulic tappets, cam-shaft, timing chains, manifold flaps all spring to mind
Best have it looked at soon as possible