TDI PD 115 Whining Noise Below 5°C

Hello,

My Gal TDI 110 PD makes a squealing noise when it's cold and the outside temperature is below 5°C. The squealing only occurs when the engine reaches idle speed, both when the car is stationary and while driving (during gear changes). Every time the idle speed is reached, it squeals for about 1 second, only once. If I give it a short burst of acceleration and then let it idle down, the same thing happens. The squealing is loudest under the car, somewhere near the turbo. There are no noises coming from the timing belt side (tensioner pulleys).
It has not yet been precisely located. It only occurs for a short period. It disappears when the engine is warmed up or after a short drive of approximately 500 meters.
Damage to the turbocharger or a defect in the variable geometry turbine (VGT) adjustment, or...?

Best regards,