I have heard this humming noise before from a 2002 b6 a4 we took it to the garage and they said it was the steering rack which was leaking therefore creating the humming noise when turning. My advice would be check your power steering fluid and see if it leaks or not and if it does leak I would check the condition of the steering rack and go from there, hopefully it might just be that the power steering fluid level is low. I hope it goes well :)