I'd try removing the plug wires one at a time while it's clattering to see if that affects the noise.  If it does you should at least have the issue narrowed down to a cylinder.  The only other thing I can think of to look at would be the rockers - making sure that all the valves are pushed down the same distance when the clattering is present.  I think you've eliminated the heads as the cause so it either has to be something deeper in the valve train (cam, lifters or push rods) or something else that moves.