I had return covers off both units. Lotsa cold air in the return plenum. Didn't remove the bottom plate yet, but will next weekend (left the coach in Reno for the short week & back to Auburn via toad).
What I did find was a massive jumble of wiring stuffed wherever it would get out of the way long enough to slap the cover on. Airflow will always generate noise, but turbulent airflow (i.e. has to scramble itself around sharp corners & obstacles) makes more. I snipped at least 6 feet of braided ground wire off & made a shorter ground pigtail. That's 6 feet less coiled up wire for the airflow to fight. Some zip ties and maybe a tie-back or two should clear the return air path some. I'm gunna take some noise readings to see if any of my fussing is worth the effort.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project