The rattlesnake has been beheaded. Yay.
I took our 2018 E450 Ford Class C to a Ford Fleet Service center. They had not ever heard of this problem or actually experienced it. But since this discovery I have now confirmed that this seems to be the problem and solution for many E450 and E350 chassis.
Serpentine belt and Idler/Tension Pulley. They replaced pulley first which alone did not fix the problem, but they also replaced the serpentine belt. Noise is completely gone. I drove it hard for 90 miles following the service and can confirm that the noise is completely gone, gone, gone.
Prior to the repair, the noise was getting worse during our previous +3000 mile trip through hilly South Dakota and Wyoming. It start out as a noise, only at 62 mph just before shifting into the final gear. As you might imagine we often hover at this speed and it was very annoying. I either had to drive faster or slower than my prefernce to avoid this obnoxious noise. During our last trip, the noise was prevalent before each gear shift and not just the last one.
But now after test the solution through all aspects, underload, up hills and heated vs. cold, all is fixed and I am happy happy happy. I love the idea that we found root cause and the solution was covered by warranty service (Less than 36K miles or 36 months)