When I start it up the engine has a mild knocking noise for about 5 seconds then quiets down and goes away. Is this normal, or do I have a bent pushrod or something? I have a 2001 SS with 30k miles on it, and the oil pressure is normal.

 

..could be piston slap.

this is a GM defect...bad rings and pistons cause this very common problem...

 

So is this something I need to fix or just have to live with?

 

depends on your time/ priorities. most likley piston slap, but it will get progressively worse with time.