I still have AFM on my G8 and I replaced my AFM and standard lifters with new GM afm lifters and GM LS7 lifters and I am still having the same problems that you are describing. I am wondering about pushrod length now but since the lifter swap didn't change the sound, I am stuck. As you mentioned in another thread, piston slap is a possibility but it seems somewhat unusual as when I had the heads off, the cylinders were still crosshatched and immaculate and the pistons didn't wiggle in the bores.