Hi - We also have an 2019 A3. I've just checked both cars and no noise comes on full lock steering at slow speed. Apologies as it looks like the steering is an electro mechanical one, so no hydraulics with bypass involved. Some cars have an hydraulic electric pump which make the humming noise. I'll get back in my box.
On full lock it will need a bypass system and the humming noise may well be the bypass in operation. Our VW has an electric PS pump and it does make a noise on full lock.