Humming noise from my audi A4?

[OM]

This problem is more pronounced the first time i start my car during the day. There is a humming noise that comes from my car when i start it and when I turn the wheel. This problem becomes less as the day goes on and by the evening, it is usually gone away. It is also worse when it is cold. TY IA

[LG]

Possibly the air con gas level has dropped.