A steady deep hum heard by the Safeway on the Mt. Baker-Columbia City line appears to match the Darigold plant just south of the store. The sound was described as lower and more constant than train noise, closer to a large compressor or diesel engine running through the night.

That kind of industrial noise can seem much louder after dark, when traffic and other background sounds drop off. In this part of South Seattle, a mechanical rumble can also travel in odd ways, making a fixed source seem broader and farther away than it really is.

Cloud cover, fog, and still nighttime air can all make low-frequency sound more noticeable. The same effect can bring in other familiar noises from farther away, including crossing bells and train horns that barely register during the day.

For nearby homes, the result can be a constant overnight vibration out of proportion to the distance. What comes across as a mystery machine spread across the neighborhood may simply be one industrial source carrying unusually well in nighttime conditions.