June 19, 2024:
Low frequency, felt not heard, felt as rumble and vibration. Truck engine. Or: some fool's subwoofer at deafening wattage. Or: the first few seconds of a large earthquake, so that breathing stops while waiting for an outcome.
This was my Loma Prieta, when the outcome was not kidding around. My living room was destroyed, the landlords gratifyingly terrified, bridges and buildings collapsed and 63 people killed.
It's been every passing truck or subwoofer since.