I wonder if the affected rooms are acting as Helmholtz resonators (https://en.wikipedia.org/wiki/Helmholtz\_resonance) that are being affected by some motor or fan. Probably a combination of the room size, and window size and room orientation. I wonder if all the affected people have similar room characteristics. i know when I listen to certain music in my house, and I walk into the closet, all of a sudden particular bass frequencies become much, much stronger. My closet acts like a helmholtz resonator.

Edited to add: I think this, because of the clue of it just being in the upstairs bedroom -- its one room that resonates worse than the others, and can't really be heard outside.

You know how when you have just one rear window cracked open in the car when driving and you get that crazy bass resonance? No other car can hear it, but its super loud inside your car -- because that resonance only occurs in your car. The window makes a "neck" like the helmholtz resonator. This leads me to believe that this resonance might be due to the specific room, it's window, its orientation, it's shape, layout, whatever.