I don't know if this in any way helps, but I just started to have massive de cel popping, it started a week ago. after checking over all the rubber bits and suspecting everything else. Yesterday morning when I started it up the sound was not right. Running my hand around the headers found the problem. What looked like a small crack in the upper header. it was very hard to see. In fact it would have been under the small heat shield at the top of the head (which is no longer there)
I pulled it off and there was a small hole and a thin but long crack winding more than half way around the casting near to the header flanges.
it is out for welding today. I expect this will be the end of the popping.

05 speedmaster - 1100cc, 11:1 racing pistons, Carillo rods, thunderbike cams, ported and polished head, 2mm over intake and exhaust valves, Barnett kevlar clutch, scepter pipes, oversize manifolds, 45mm HSR's, TTP stage 4 firestarter