Automatic Voltage Stabilizer at work...
Occasionally I'd hear what sounds like a pumping/sucking through a straw sound at our refrigerator. Turned out it's the AVS at work. Not surprising given the poor quality of power service here in the Philippines.
An AVR is high-recommended most especially to protect critical appliance from voltage surges when power returns after a power failure.
The AVS delays turning on the power to the appliance for several seconds. It is hoped that those seconds would be enough to let voltage stabilize before powering up the appliance.
I THINK the servo system mechanically moves a part to adjust inductance or coupling of integrated transformer.