There is a hairline crack on the manifold. Although it sounds like a metal rattling noise, it is more likely the sound of air being forced through the crack. It was interesting how we even found it. With the car in park, I hit the gas and there was none of that sound. The sound is audible whent he car is in motion. We tried another test. With the car at rest, I put the car in "D", left foot on the brakes and hit the gas. Eureka! The noise is audible and it is near the passenger firewall. With the help of a mirror, my mechanic found the crack. And the dealer's diagnosis was the catalytic converter!