T@B 400 Front Storage Box Noise

We recently took our 2023 400 on its maiden voyage, which towed behind an AWD Honda Pilot just fine... except for one annoyance. There is a frequent metal clanging noise coming from the camper once we get up to speeds above 35 mph, and very frequent at highway speeds. It appears to be coming from the metal storage box, but I have yet to successfully even diagnose it yet, much less pursue a solution. Among the things I tried:

• Took everything out of the box, in case something is jumping around in it
• Damped the interface between the hinged lid and the box with towels
• Damped the tiny gap between the top of the propane cylinder and lid, again with towels
• Removed the propane cylinder entirely
• Drove with the lid open rather than closed

Same noise, every time, which may indicate that its coming from the bottom of the storage box, or even elsewhere. Has anyone else had this problem, and if so, how did you address it?

Thanks in advance. Apologies if this has come up before, but I couldn't find it in a search.

Tom