What is that strange sound on the A320 after the doors are closed?

Hi Capt. Lim,

I fly frequently on AirAsia A320. I observed there is a strange sound right a few moments after the doors are closed and armed. It sounds very loud like some motor whining. What is that sound?

I know it’s not the flaps lowering as it doesn’t sound that loud.

On the Boeing 737s, there is no such sound.

Thanks.

Best,

George