Here is a thought,,,

Can you make it do it consistantly? If so, grab a insulated plyers, and individually remove plug wires as its idle/running, this might pin point the cylinder/cylinders with the issue. I have heard worn rocker arms make a similar noise. Do you run the good ol trusty stamped steel OEM rocker arm? I had a 318 that would rattle at idle, and sometimes under load if you knew how to do it. You can take an old valve cover, cut the top off, install it so you dont get oil everywhere, and put slight preasure on each rocker with your thumb. AT IDLE ONLY!!!, and you might be able to make the noise go away.

Kasey