The sounds very much like a relay switching very fast. If this is done with a considerable load to the supply, it will relatively fast damage the relay contacts and maybe other parts. So this unit is broken and should be returned.
Typically such supplies have a relays or two to switch between transformer taps, depending on the output voltage. Normally there is quite some hysteresis to prevent to fast switching. Here the hysteresis is missing or way to small - likely something like a missing resistor or solder joint / broken line.