We use an electric water kettle, made by Hamilton Beach; its capacity is
1.7 liters, electrified through its fixed base plate supplying
1.5kilowatts. The shape is a truncated parabola of revolution, focus up.
When half filled, it shows a distinct acoustic pattern when the heating
produces gas bubbles. The sound oscillates on a 10 second periodic time,
from silence to a quiet pink noise and back, never louder than 60 dBm. I
suppose there is a positive feedback loop of some kind. Care to
hazard a guess?