Sounds like a "standing wave" phenomenon, where sound reverb is bouncing off interior surfaces and colliding in a spot where it is additive. I took some noise measurements in my 06 when working on the AC noise and found the interior to be fairly "hard," i.e. noise hits the walls & windows & bounces back. In cases like that you can double or more the perceived sound because you get it going by, then again bouncing back, etc.
Pull the AC vent and check three things- first how tight are the screws holding the AC unit down to the donut of foam that forms the seal? If they are cranked down real hard the compressor vibration will drum on the roof. This is easier to make happen in the front than the rear where the roof is supported by walls closer than just the sides of the coach. Loosen the screws a bit at a time and see how it affects sound. You can re-tighten as needed to maintain a good seal, but sometimes an RV tech will resort to WRV'ing the screws (you can do a search for the technical meaning, basically put 400 ft lbs on a 40 ft lb fastener).
Second, dress up the wads of wiring and fasten it out of the way of the air path. This dropped 3dB for my 06. I also moved the electrical box out of the airpath; basically anything that will require the fast moving air to buffet around it will make noise.
Third, check to see if they properly aligned and taped shut the interface between the down-duct and the fore-aft plenum. These should be sealed so there is no leak of cold air into the return air path. The return air goes up and around the plenum into the AC unit's cooling coils then comes back down the down-duct. If left untaped (HD & Lowes have aluminum tape just for this chore), the cold & return air mixes in rambunctious ways making noise. Not to mention you lose a lot of efficiency mixing your cold air w/the return air.
Hope this helps.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project