Most likely piston slap. Maybe--OP hasn't provided a vid or audio example.
Wrist pin noise [ worn pin &/or pin bore ] gives a double tap. <This noise won't go away when warmed up
As long as the rest of the ign curve is altered to suit, running 20* initial is a good idea. Engine will run cooler, use less fuel & have better tip in response. The GM LS engines have a very good combustion chamber, only need about 27-28* @ WOT. They idle with 22*.....