Wait a minute, you might not need to whip the head off just yet! I have a feeling that you may have an injector problem, and it is certainly something you can check before you whip the head off (and it is good practice to take the injectors out before you whip the head off anyway).
First of all, if you build up some speed in fifth gear and allow the vehicle to coast at around 45-50 mph on a gentle descent, do you notice the rattle more as you come off the throttle slightly, with just enough throttle to allow the engine to coast and maintain speed?
Also, would you say the rattle is fairly consistent / does it maintain a steady pattern, or is it more random?
The black oil that you see on the glow plugs could well be quite a few things. I would like to suggest that it could be soot / carbon build up which is created by an injector over fuelling in that particular cylinder. The injector may have a worn nozzle or spring which is allowing fuel to leak into the cylinder when it should be completely sealed off, or it could even be a leak between the nozzle holder and the nozzle it's self which is allowing fuel to dribble into the cylinder. The excess fuel in the cylinder will create pre-detonation (diesel knock) and it would also explain the wetness on the glow plugs (diesel and soot build up combined).
If you whip each injector out of the head, examine the lower part of the injector (I.e. Around the nozzle and copper washer area) for wetness. If you find one that is damp (they should be dry), you've got a leak and it may well be causing your knock.
You can check each injector by connecting the injector pipe the pump in a way that allows you to position the injector so that you can examine the spray pattern. Turn the engine over (on the starter) and examine the spray pattern. If you've got an injector that is dribbling diesel, there's your problem. If you notice an injector body leaking, this could also be causing the problem.
It's worth having a look
-Pos