2002 Audi A4 HUMMING NOISES

I have a 2002 Audi A4 w/ approximately 62K. A few weeks ago, a slight humming noise began when the AC was turned on. It would hum for a minute or two, then the car would be quiet for the rest of the drive. For the past 2 weeks, the car hums continually (even if the the AC or heat is off).

There is no noise when it is started and in Park, but as soon as I move it in another gear, the humming starts. The humming seems to get quieter if it is driven more than 50mph, and it also seems as though the humming itself is progressively getting louder (especially if the car is idling or being driven at speeds less than 50mph).

Everything else on the car is working as normal. I plan on taking the car to a mechanic over the weekend, but I would like to have some sort of idea as to what the problem could be beforehand. I have read on a few posts that it is either the wheel bearings or the power steering column? Does anyone know what it could be?