we recently had the plastic inside air conditioner vents (covers) replaced with a decorative, upholstered cover. they sealed (taped) off the air outflow portion with an A/C vent material that looked like a metallic piece of cardboard. that out flow air is now forced through the various round A/C vents throughout the cabin. This significantly reduced the noise level inside the RV.
If I am reading your post it sounds like the quick cool vent on the bottom of the A/C is open. Does most of the air come straight down from the unit? If so close the slider that then forces the air to the other vents and will reduce the noise level. You don't have to do anything expensive just close the quick cool vent. It sounds like that is exactly what they others did except they closed it off.