The best way I can describe the sound is if you were to wrap one knuckle on a small piece of wood. I just did it and it's only slightly higher in tone than the actual sound the engine is making.

 

Yes thats what I'm talking about,,

 

So you think it's carbon build up?

 

It shouldnt be that much carbon with those miles and the carbon cutter valves.
Take a small hammer and crawl under the truck and tap on the cats. It is rare, but the cats going bad can make it sound like lifter/rod knock.

 

If the people before you used cheap gas yes,, you would have carbon on the top of the
pistons and that will raise the compression just enough to kick in the knock
sensors to stop the knock.
RXP is about $7 so its a good place to start if the noise is still there
after then keep looking.
I always start with the easy things to see if it helps, thats why I
also said to change the plugs.
Good Luck.

 

So where do you buy this RXP?

 

AutoZone, Pepboys its a 2oz bottle thats orange and will treat 25 gals
I have used it and found that it works, have seen it when doing emissions
on a Dyno with the NOX reduced.