I had a thread going a while back on this. You may want to find it.
My '02 SS Camaro starting ticking at about 20K, as you describe. Endless subject on SS owners forum. Took car in at 35,805 miles before warranty expired. 4 of 8 cylinder walls out of round by .002 top to bottom. got complete engine, not just long block but fuel rails, ps pump, etc..
normally, the slap is heard easiest / most on #1 piston
was told pistons now coated with teflon/nylon coating and that at 20-25K coating WOULD wear again. not if, or maybe, or could. this chevy dealer sponsors my nephew racing, so were close to them. at 22K on the new engine, slap was back as i was told it would be. The SS was daily driver
there's no fix to piston slap other than rebuild and sleeve, but more of what degree of it you'll get and when. some report no slap at all but then alot of fbod, c5 owners only put a few mile on the cars
my other thread's got a link to a website about it. i predict this a big issue with other goat herders in the future as cars get miles on them.