Gee, that's mighty subjective. I have a neighbor in the RV park with a newish Alumiscape (Holiday Rambler) 26-footer with a DuoTherm 15K BTU, and it's much quieter inside than the antique Armstrong in my 31-foot Airstream or the Carrier V I put in my 28 Argosy.
That said, I ran the Carrier full blower just to get the Argosy cool in the summer. The old Armstrong can do the same in the middle fan position, making less noise overall even though the compressor is much, more noisy.
My GUESS is that Airstream products are just as noisy inside from the A/C as any other brand. The shape of the roof make it shed a little noise, but not that I can notice.
I'm just curious about why my neighbor's Aluminscape, which has been across the country and back three times, is so much quieter than BOTH my Airstream products.
Of course, my trailers have been around 27 and 31 years... compared to these, the HR is a baby.
PS - The advantage of the 31 is that you can get further away from the A/C, and with the curved ends of the trailer, still get plenty of cool air where folks lounge and sleep. The center beds of the Argosy did not show this effect. By the time I got the entire trailer down to a comfortable sleeping temperature, the bathroom would be frigid.