Forgot to mention the knocking speeds up as the idle speeds up and slows down as I let off the gas and idle slows down.
This sounds like rod knock or piston slap. A definite possibility. If you've always had the truck, have never oil starved it, and don regular oil changes, it's near impossible to have rod knock/piston slap though, so that's very strange. And you would never get a check engine light/code from rod knock/piston slap, there's no sensor for that.
EDIT: Could still be valve tap though, valve tap will increase speed with increased engine speed. Sounds like you know the difference though....valve tap vs rod knock/piston slap. valve tap is more rapid also(more times per revolution than rod knock/piston slap)