I have a quite a distinct knock when the motor is cold in my 99 z28. Some have told me its normal.. and others say get it checked.
What mit this be.. just a general guess.

 

if it is only when its cold and goes away after a minute of running it is most likely piston slap... not a lovely sound but tons of ls1 suffer from it... doesnt seem to affect anything just kind of embarrassing starting up your nice car and it sounds like a diesel lol...

what the problem is is the pistons gm used in the ls1 are a short skirt piston for low friction but some of the pistons were undersized a tiny bit, and until they warm up they rock in the cylinder making that terrible noise.

 

when it get warm it goes away almost positive its piston slap i think i only hear 3 ls1 not doing it and they where 02 with LS6 blocks

 

could be lifter noise..mines been doing it for the past 10k miles, no other problems