Alot of current GM engine suffer from piston slap. See
www.pistonslap.com for some insight. We get the sound which does sound like a lifter in my son's '02 Olds Alero GL2 with the Vin "E" 60 degree 3400 engine but it disappears after approximately 60 seconds. Not much you can do about it if it is piston slap. GM changed the composition of pistons and engine build technique. My experiences with bad lifters is they stay noisy and get worse when the engine fully warms up.
But to be sure I'd double check the rocker arms and re-torque.