Tom P
I'm Awesome
If you have a knock at start up that goes away after a minute or two, its probably from excessive carbon buildup. Had my 2001 K1500 in the shop for a dead fuel pump issue so I asked the guys at the Chevy dealer why it made that issue and they said it was extremely common in the 5.3L engines and was usually caused by excessive carbon in the combustion chamber. If you are experiencing oil consumption then its very likely that that is causing the carbon build up, thus leading to your knock or piston slap, or whatever you'd like to call it.