Piezoelectric effect from capacitors? I have had an original sound board with a singing ceramic disc capacitor. If you listened close you could even hear similar noise that was coming out the speaker at the same time.
Some switch mode power supply circuits make noise when the power is turned off. Seems like the regulator is kicking out of normal frequency repeatedly as the capacitors drain out and an audible clicking noise is heard for a few sounds. If its fast enough i guess it would be a hum. I have some 12v power supplies for LEDs that do this at power down.
I wouldn't worry much about it.