I found the cause: the wheel cover is wobbling pretty badly as the wheel rolls. It's off center or out of balance. The whole thing looks something like Kaper II : Pop-Up
You can see the hubcap in the middle of the A5USC-T picture. It's not exactly what's on the Damon, but it's close.
Now... how to fix it? Dunno. it's on there firmly. 24 month MX is coming up at freightliner, so maybe just let them have a whack at it.