[Chevy-SS]

If it's like a tapping or clicking noise, then probably a lifter. If it's heavier knocking which goes away after engine warms up, then it's probably piston slap, which is common for Chevy engines. If it knocks (heavy) all the time, warm or cold, then it could very well be a rod bearing or possibly just bad piston slap. The piston slap will definitely decrease as the engine warms though, so if it's piston slap, it would be very loud at cold start, then mildly loud (or quiet) after warm.