My '06 Dmax started to clater & smoke briefly as I would drive it about 30 to 40 MPH.
It does not do this all the time. It also idles rough ocassionally.
When it smokes. It only last for a few seconds after the clatter noise.
The noise is not a hard knock.
If I rev up the engine in Park after this has happened it won't clatter or smoke.

I had the same issue at idle too. I noticed it with the exhaust going from the hiss sound to a deep lb7 type sound and noticable smoke coming out too. I pinned it down to when the a/c compressor cycles that's when this happens. Also the compressor makes quite a clunk when it engages maybe this is what you are hearing.

The AC was off during this.
What does an injector do when stuck open or closed?

Was your defrost on? AC compressor would be on during that

Hot many miles, any other issues and what mods do you have on the truck?

It is the 6.6 liter w/ Allison tranny no mods and has 47k mi. and all conditioning was off.
The cloud of smoke starts off white and then goes black.

EGR running tests ?

I had the same issue at idle too. I noticed it with the exhaust going from the hiss sound to a deep lb7 type sound and noticable smoke coming out too. I pinned it down to when the a/c compressor cycles that's when this happens. Also the compressor makes quite a clunk when it engages maybe this is what you are hearing.
What you are hearing actually sounds more like the veins in the turbo opening and closing.