What is knocking on a small block Chevy?

Answer 1

Could be lean fuel mixture or excessive timing causing pinging or pre-ignition. Could be bad bearings on the crankshaft. Could be worn cam or rockers. Could be broken rocker stud. Could be exhaust manifold leak. If it goes away within a couple minutes of starting, it's possibly something called "piston slap". That's when the piston is slightly worn at the bottom resulting in enough space for it to pop back and forth in the cylinder. Once the piston warms up it expands and the piston slap goes away.

Another possibility is rod knocking. If that's the case you'll have low oil pressure. Sometimes a thicker grade of oil will help that problem.

Another possibility is lifter noise. That's just a matter of sludge in the lifers or lifer oil lines.

Another possibility is the "temperature riser", which is nothing more than a tube across the intake manifold and exhaust from one side of the engine is diverted through the riser until the engine is warmed up. Sometimes the valve assembly will make noise.

I'm sure there are other possibilities but try those for now.