Attention: TDR Forum Junkies 
rbattelle
TDR MEMBER
What follows is my opinion only, and should therefore be taken with a large bowl of salt.
In my opinion, the reason your reception from Cummins and DC has been relatively cold is that you're chasing after a problem that really isn't a problem. Without having listened to your specific engine, I believe I can imagine the "spark" knock - does it sound like "ping" in a spark ignition engine?
If so, this has been around since the dawn of the 3rd generation trucks.
Furthermore, in my opinion, this noise you're hearing has been going on in your truck from day 1, but as it breaks in and, perhaps more importantly, as you continue to obsess over it, you perceive it has become more pronounced.
Thread from 2004
Definitive thread on the subject from 2004
Knock with sound clip (great thread)
Ping noise
Another thread on ping
Rattle at midrange RPM
The obvious question is whether this can be fixed, and more importantly, is it something that needs to be fixed? I suspect the ping might be corrected by using more ideal injection timing. Ideally, peak pressure occurs about 15° ATC, so that the delay period occurs across the top-center portion of the stroke. However, I believe Cummins intentionally retards the timing a bit in order to (theoretically) get combustion to run longer (for more completeness). You might guess, then, that by advancing the timing a little bit you'd pick up a little efficiency, and decrease the noise (at the sacrifice of emissions).
Of course, multiple injection events throw a wrench into that whole idea. If you're giving a pilot shot very early in the stroke, you've already 1) consumed some of the available oxygen in the mixture, and 2) significantly raised the heat in the combustion chamber, which should significantly reduce the ignition lag when the main fuel charge is delivered. The pilot injection, then, would have the effect of quieting the engine significantly (as we know it does).
Is it possible, then, that the pilot injection itself is the actual cause of this knock? It's certainly possible, and you can imagine that the knock would be quite small, since the fuel delivered is quite small. The cylinder would be relatively cold when the pilot shot is injected, and if it's delivered very far BTC, the rate of pressure rise would likely be extremely high, resulting in noise.
These are just some ideas, I've not done anything other than qualitative analysis on this, so it's just a theory.
Is this a problem, then? Will it harm engine durability? Yes, any kind of detonation will decrease engine life (in fact, to be accurate, simply running the engine at all will decrease it's life - maximum engine life is achieved if the vehicle is parked and never started up). The real question is, how much? If the pilot injection is what's causing the knock, we're talking about very small amounts of fuel, and very low energy relative to the design loads for the pistons, rods, bearings, crank, and block. Imagine what the peak pressure is from a shot of fuel that's only a small fraction of the design fuel charge.
If you want to talk to engineers at DC or Cummins, and get anything useful out of the discussion, I strongly suggest first reading the relevant literature on combustion in the diesel engine. A good place to start is the NACA technical reports server.
Ryan
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Why would you lubricate a belt?
I poured the can down the throttle body while holding the throttle assembly to keep the engine running. At first, there was a cloud of white smoke at the exhaust. After about half a can, the white smoke was gone. The truck ran very well afterwards. If you read all the uses section on the can you might think the stuff could cure cancer though:-laf
It's definitely "slick". :-laf 
