[DK on the road]

I would tend to think it your onto something with the Regen system. I've been around Semi trucks and heavy equipment with all different kind of regen systems. Some did make noise to keep them cool, even before they were hot, because they create extreme heat during the their processing. Your regen would need to be on the driver side rear between the Radiator and engine on the exhaust system. Most systems only go into "noise making" or are activated once they have hit some criteria, for example, Hour meter, build up on a sensor of certain particulate. They must complete their cycle as from what I've seen.

Can you send pictures of what you think is making noise, maybe it can be identified that way

My coach is prior to regen, but I have the C15. Not sure what is the first year of regen.

Keep us posted on what you find.