The nose and main landing gear hydraulic systems were connected to the hydraulic pump and power was supplied to the hydraulic pump bus. The emergency gear operation switch (in the pilot’s emergency panel) was wired up to allow testing of the gear retraction and extension.
As the video shows, the hydraulic pump pressure relief valve opens before the high pressure set point on the solenoid switch is reached. The result is that the hydraulic pump continues to run after the gear is up, and the “screeching” noise is the pump’s DN-side PRV opening.
Further investigation is required.