Grubby John I think the ends of the shjafts are flexing without end supports when hot. The shaft could be cracking and you wouldn't see it until they break. Yes I have been through this. Get a set of end supports. You said the noise is only at the ends. This is where my shafts braoke at the last bolt hole through the shaft. Some one sells them here. If you can, pull the noisy lifters apart and clean them out and reassembly. Lubed them with pre lube and drop back in the correct holes. I wonder if the shafts where unhooked. When my motor sits over the winter, I loosen the shafts to release the valve springs. This takes the pressure off the lifters. Peoblem with this is when I was running anti pump lifters(non rollers) I had to remove all the lifters and drain the
oil. Then reinstall them and set the rockers assemblies to torque spec. Fire up the motor and the lifters adjusted. If I didn't do this the valves wouldn't close and motor ran like crap. These where crane antipumpups from the late 90's. This could be the noise. Run the motor again and put pressure on the end of the shafts and see if noise is better, worst, or the same. It could be just the rockers, swap them around and see if the noise moves. If it does, its rockers, if not lifters or shafts. When shafts off, take straight edge and make sure that all 4 mount locations are flat, if not may have to shim them. Rick L.