why has my engine started knocking

Just saw this string as I was replying to a similar string I've been commenting on since '09. Here's a cut & paste of my explanation from '09 based on my own experience. Your problem description sounds consistent with this. My info at the time indicated they had done something to "fix" this on the '03 models & beyond, but perhaps it was a mid-year update if yours has this issue. I had over 100K on mine when I sold it & still running OK otherwise (burned a quart between oil changes) and I've heard of others with far more miles than this. "My 2000 with 5.3L does the same, and has for over a year (has 86K miles on it now). There is all kinds of info posted on the internet about it. Apparently there is a bore distortion or piston/cylinder clearance issue when the engine is cold (aggravated by the short pistons on the 5.3L), particularly after you get some miles on the engine, and what you're actually hearing is piston slap. The odd thing in my truck is that I hear it better in the cab than from outside. Don't know if that means it's on one or more of the rear cylinders, or just has to do with how the noise is transmitted through the structure. Annoying as hell, but short of a new engine there's not much that can be done. I think mine has gotten more noticeable in the last year - cold weather is the worst for it. GM naturally claims it will not be detrimental to engine life. I'm way out of warranty, so I'll drive her til she pukes one way or the other. Do an internet search on 2000 Silverado piston slap, you'll find a lot of people with similar experience. Apparently they did something in the '03 & newer models to get rid of this issue."