Is the engine recently rebuilt, have new heads, solid tappets/lifters? Did the ticking start out of the blue? Do you have a new exhaust system on the truck?
For hydraulic tappets/lifters, it looks like the other postings have covered things pretty well.
I think I read that you have manifolds. If your still looking at the exhaust, besides checking the manifold to block you might take a look at the manifold to exhaust pipe. If the donut is cracked, deteriorated or the connection is not tight you could get a ticking noise. A leak from any joints in the exhaust, especially if there are slip joints on the exhaust system.
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