ok this is my friends 1988 firebird. When he starts the engine you hear a knocking like its the valvetrain or something. it goes away after about 30 seconds. it only happen when it below 40 Degree out. any idea? it a 5.0L fuel injection engine all stock with only 30k miles on it. thanks.

It's most likely piston slap, small blocks are notorious for it. The clearance between the piston and the cylinder wall allows the pistons to "slap" the cylinder wall making a knocking noise. Once the pistons warm up they expand and the noise goes away. Nothing to worry about, just try to avoid racing the engine when it's cold.