Flathead V8 Idle, Timing-Gear Noise?

What's this noise my car's engine is making? To me the clacking sounds like it's coming from the aluminum timing gear. Any ideas?

This is a stock 59L Ford flathead V8. It's a fresh rebuild with fewer than 4,000 miles on it. It starts instantly, idles smoothly, pulls strongly and even gets great fuel economy, but it makes this weird noise at idle. It gets quieter when the engine is fully warm, nearly going away.