Why do I hear a knocking noise when I start or turn off the truck

Many possibilities that can't be diagnosed over the internet, have *mechanic* listen to it!

Pull-a-part in nashville is about to be your best friend. that year had sleeve problems. the sleeve drops either slowly or abruptly. if slowly, the noise you hear is excess air in the cylinder. gm has a recall on this, so call them first, if that doesnt work, youll have to replace the engine. if it were me though, id find other transportation, pull the motor and put darton sleeves http://www.darton-international.com/sl_resleeve.html, port the heads, machine the block a tad (go to TCAT, that community college by the mall), arp pistons, and upgrade every bolt, nut, and fastner. of course doing this requires more fuel, air, spark and exhaust flow. but im a gearhead. scope out someone doing some major repair work around mtsu. those houses behind it house plenty of would be mechanics. personally, id use their help, but not let them lead. that pull a part sells engines for really cheap and you can find around 50-100 people a day there.