My 2000 Corvette LS1 makes a tapping/knocking noise 15-30 seconds after cold start that fades in about a minute — what's wrong?

A noise that shows up cold and fades within a minute as the engine warms is usually not rod knock — rod knock doesn't change much with temperature and gets worse with load. The likely suspects:

• Hydraulic lifters that bleed down overnight and tick until oil pressure fills them.
• Piston slap: pistons rocking in the bore while clearances are largest. It's loudest on a cold start, sounds hollow and dull, and goes away as the engine heats up. Mild piston slap is generally considered an annoyance rather than a real problem.
• An exhaust leak at a manifold gasket, which can tick when cold and seal up as it heats.

Quick checks to tell a lifter tick from something worse:

• Where it comes from: a lifter tick is up in the valve-cover area; rod knock is deep in the block or oil pan. A long screwdriver (tip on the valve cover, ear on the handle) or a mechanic's stethoscope helps.
• Cold vs hot: lifter ticks are often loudest on a cold start and fade as oil pressure comes up. Rod knock doesn't change much with temperature.
• Load: rod knock gets louder under load and with rpm. A lifter tick is usually most obvious at idle and may quiet down with rpm.
• Kill one cylinder at a time: if the noise drops out when one coil is unplugged, suspect that cylinder's rod bearing. A lifter tick keeps going because the valvetrain still moves.
• Rule out an exhaust leak: a leaking manifold gasket can sound just like a tick, especially cold. Look for soot at the ports.

If it ever starts knocking hot, under load, or with low oil pressure, stop driving it and investigate. More: piston slap vs lifter tick vs rod knock.