Moisture indicator honks when I plug in the phone

My Samsung Galaxy Note 8 honks and buzzes when I try to charge it — about once out of every four or five times.  It says moisture is detected, presumably in the charge port.  There’s no moisture or dirt – I’ve cleaned, scraped, done little bits of alcohol on a cardboard corner.  I always need to restart it before charging it to make the noise and warning go away.  It does this with all my cords – at home and in the car and at work.  It does not do this with my inductive charge pad at work, but I don’t have one at home or in my car.  Any thoughts?