Why does your kitchen faucet squeal when turned on?

As far as I know, its usually a loose washer in the piping somwhere or a mishapen one. An old plumber told me if you hammer the washer on flat surface, that can correct it. Basically as you run the water it resonates creating the noise. The washer is probably in the faucet btw. Like I said I'm no expert just saying from experience.