Also trying to figure out the solution.
To me it looks like an "inner" airbox inside the regular one. So basically an extra chamber made of a rubber car mat

Interested to see what comes of this solution and if it becomes a workable DIY for others as it appears the RX4 also has the loud intake symptom most prevalent during high load situations, ie, accelerating up a highway grade

for those interested I will explain what I did and how it works the walls in the filter box are lined with new rubber because it is the safest material, it does not crumble and does not dust the rubber absorbs sound bouncing back from it so you should lining the walls as much as possible on the outside the box is covered with sound absorbing sponge but the biggest effect is the modification of the outlet

1) a rubber chamber to reflect sound going from chamber 2.
2) A rubber chamber that absorbs the sound going directly from the throttles.

1) original outlet plugged with rubber.
2) New outlet.
3) A wall made of rubber.

I can see from the pics that the RX3 airbox differs from the RX4 but the premise should be the same.. if rubber and foam helped quiet it down that should also be the case on the RX4 if it is indeed the airbox that is the noise maker under load (especially annoying when i wear a full helmet, hence have been using my half-helmet mostly)

ok any question you want to i record the sound of engine or something?

As a percentage, by how much would you say you were able to quiet it down by with your mods?

If someone is not sure whether the sound is coming from the air can, unscrew the seat, start the motorcycle, plug the air intake with a sponge or a towel and accelerate a few times, this should dispel any doubts and unfortunately the motorcycle muffler is not very quiet, so I ordered a new one.

ambient sound with engine off is 30db in neutral gear engine 0n the app shows 77 db at max throttle 89 db when driving below 6000rpm you can't hear the engine above 6000rpm you can hear the bass (instead of the metallic sound that makes your teeth hurt) generally the rubber walls dampen and modifying the exhaust by making a maze for the air noticeably reduces the sound you can check by unbolting the seat and plugging the intake

I have a sticker (that i removed) on the RX4 that says 80db at 6k

I use the application from the phone and you cannot trust it too much, especially since it believes 30db with the engine off unscrew the seat, start the motorcycle, stick the intake sponge or cloth and see the difference and let us know how you can

you can also measure the sound with the phone your engine idling and open throttle and see what the difference for me is 12db

I decided to do the test again I locked myself in the garage I used the same app as in this videohttps://www.youtube.com/watch?v=7ONhvaDBds0 I took pictures with the engine off at idle 4000RPM and at 6000 RPM here is the result.