Learned the hard way that the lint trap is only half the story. I used to just tell people to clean it and move on, but after pulling enough packed vent lines I started checking the outside flap before I even touch the machine. Now I take a phone pic of the clog and show it to them while I hand over the bill, because arguing about it just makes them dig in. Nine times out of ten they go quiet once they see the fuzz mat, and the ones who don't are the same ones calling me back next year. That eight month warning you gave them was more than fair, some folks just have to learn it the expensive way.

Did you catch it on a scope or just by pulling breakers one at a time until the noise quit? I've had a strobe pack do the same thing, and what got me was it only showed up when the beacon flashed, so the hum was really a pulse riding in on the shield. If you still have the old supply, try running its ground straight to the airframe instead of through the audio harness and see if that kills it. Also curious whether the hum was there with the engine off, since that would rule out a lot of the charging system right away.