I like the listening stick concept,I sometimes use 3/8 extensions. What H/R says sounds like a logical approach. Insure you have oil pressure and then listen to all the externals, fuel pump, water pump and do the clutch trick. Watching the valve train while running will tell you if everything is getting oil on the top and you can check your clearances quickly. My shot in the dark diagnosis is that something is banging on a cover, either valve or side cover. I don't know how this could happen, but you never know wht you are going to find. I don't know the answer to this but could a wrong sized valve cover be the problem? Is that even possible?

1946 1-ton Panel
1952 1-ton Comml. W/Grain Body