Update: My tank of 93 octane is down to a 1/4 tank now and it is exhibiting the same behavior some of yall mentioned earlier, where it idles low around 600 rpm, and goes back up to 700 rpm when AC is on. Wasn't doing this before, but now it is. I wonder if Honda has a fix for this? I am still waiting to have time to try and clean the intake, and take it to dealer.
So I've had this 'issue' since the car was new. It happens a lot less when I use ethanol-free gas, however that's due to the rev sitting at 700rpm at idle as opposed to 600rpm.
I'm an aerospace engineer however I'm fairly certain I know the reason the passenger chair rattles at 600rpm: Resonance.
I think this is a design flat between the seat and structure of the car by nature. Every object has a frequency that if it experiences, it will vibrate in response noticeably (think of the mythbuster's episode where they shattered a wine glass with that singer keeping a specific note). The only way for Honda to fix this is to add weights to different parts of the seat, redo its structure, or apply dampers to the seat-to-car mounts.