When testing speakers, I discovered my room (or house) is resonant around 40Hz. On most other frequencies the speaker was quiet, at 40Hz it felt like the room was shaking. I wonder if the effect of vehicle noise (I'm between a large road and train tracks, and near an airport) is amplified by this, though I've not noticed any impact - then again, it's not as though the vehicles are generating constant sine waves, and I don't have a quieter baseline to compare to.