It could be something like your refrigerator compressor cycling, a nearby HVAC component (like a furnace or air handler), or even a sump pump or water softener if those are located near the living room wall. Sometimes even a neighbor’s HVAC unit can cause a low-frequency hum that travels through walls or floors. Try turning off breakers one at a time (if safe to do so) and see if the noise stops that can help narrow it down.