Historically the fabled “West Seattle hum” has tended to be related to vacuum offloading of dry cargo on the Duwamish River. Every time a new complaint comes in (maybe once a month or so), we check VesselFinder and sure enough, a bulk cargo ship is in.
https://www.vesselfinder.com/
But the only way to verify that is to drive down to West Marginal and see if you can trace it. That’s what we and community leaders did more than a decade ago when this was a raging mystery – TR