When the electromagnet pulls the plunger down, it doesn't close the drain flapper all the way, (only about 3/4 of the way closed), then the water pressure hits the flapper and holds it sealed against the spray arm outlet so the water goes out the drain.

When the water pressure hits the drain flapper and opens it the rest of the way it takes the load off the solenoid. The solenoid is only energized for about 30 seconds and if it finishes draining before the the timer turns off power to the drain solenoid then the return springs will pull pressure against the magnetic force of the solenoid and can cause the buzzing noise.

Replacement drain solenoid kit

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Source: GE dishwasher buzzing drain solenoid