Sounds as if the injector for one cylender has sarted to drip slightly,
which will cause a pre-ignition knock.
If you are feeling brave, then all you have to do, while the engine is
running, (I assume you can hear this knock at tickopver?) slacken the nut on
the top of the injector which holds the metal fuel pipe. Fuel will leak out
a bit, and the engine will start to miss a little, but the knock will also
go away if you have tried the right cylender. If need be, do the same for
each injector in turn.
If this does cure the knock, then first of all try some diesel injector
cleaner through the system, and if that doesn't work you need to find a
diesel injection specialist who will recondition the injector.