I have noticed a metallic popcorn popping noise when I drive my 1992, non-turbo, 245 with 68,000 miles. If I am cruising on the interstate or up a hill and let off the gas slightly, then I hear the metallic popcorn noise. As soon as I apply the gas again, the noise goes away. But it returns again when I let off the gas. Any suggestions? Heat shields? Exhaust manifiold gasket? Valves needing adjustment? Pre-detonation? Knock sensor? The car has had new plugs, wires, rotor, rotor cap, fuel filter, air filter, timing belt, throttle body cleaned, along with regular oil changes over the past 12 months.