Your conclusion of noise generation is correct.
I've played around with exhaust silencing also and concluded the same as you.
Lining the inside of the plenum isn't likely to do much to mitigate the intake pulse noise, it's more a need of sonic wave diversion or baffles inside the plenum.
It may be a result of plenum size/shape making it louder than it needs to be. I wonder if the RC3 uses the same box and does it make the same sounds. That would require the evaluation on someone who has access to both models. cough cough Gatling cough
I also wonder if the body & windshield bits amplify the noise. Anyway, since the air filter chamber isn't something that easily pops out, I haven't bothered to pursue the issue. Now if someone decides to replace the stock system with a pod filter on the end of 4 feet of radiator hose, perhaps they can report back.
