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Squeak could be the accessory belt the knock get a stethoscope from Harbour flake and go chase it see how close you can get to it so to have a better guess also put your finger on the valve cover see if you could feel knocking could be a rocker or a broken spring.
LS3's can certainly knock a little when cold due to piston slap. It's especially noticeable in colder weather but should pretty much go away when fully warmed up.