this isnt a knock it isnt even a constant or predictable tick.... it is more like a loose bracket or mount than anything else.
Without hearing you truck in person its hard to say, but every 6.2 truck I've ever had has a distinct, erractic, ball bearing bouncing around in a tin can to them. Its not really diesel, but its kinda on top of that, iif that makes sense. It sounds like something is loose, like washer spinning on loose bolt kinda sound. The sound I'm talking about is not a loud, obnoxious sound, its just kinda there. I was convinced on my first 6.2 truck, years ago (an '84 K10) that something HAD to be loose somewhere. I put a wrench on every bolt I could get. The noise sounded it was everywhere, but I was convinced that something was loose in fan shroud/radiator support/grille area, but I never found anything loose. I hear it the most when they are good and warm and just idling. I don't even pay attention to it anymore.
I wouldn't say it sounded like the starter was touching ot anything, so your's may be something entirely different. How long has your's been doing it.