The older lady who taught me to listen to the machine, not the noise
I had a call last week in a 1960s ranch house, a washing machine that was thumping so loud the neighbor knocked. The owner was about 75, and she just stood there watching me poke at it. She finally said, "Young man, that noise ain't the drum, it's the belt slipping because the floor is uneven." I spent ten minutes checking the level and she was dead on. After I fixed it, she told me her husband used to say to listen for the quietest hum, not the loudest bang. Has anyone else had a customer teach them something simple that saved you a lot of time?