Humming sound

My initial thought was that your transmisson is the source of the noise, put then you mention the steering, it may be both. First thing to do is check the power steering fluid and transmission fluid levels. If you need to add fluid, add the recommended fluid as stated in your owners manual. If these are OK, or your unsure about how to exactly go about doing this, then take it to your BMW repair shop and have them give it a look.