Sounds like a Gnome inside your engine banging a ball peen hammer against the block. Someone may correct on this, but the cause is more or less similar to predetonation in a gasser. The spray pattern from the injector, or some other part of the combustion process gets out of sorts. The rackect can go to extremes and will cause damages if not taken care of.
Most diesels have light nailing sounds at idle. When that becomes a problem is usually easy to hear.