I bought a nearly new rental 2009 Impala, with about 10000 miles, from a Chevy dealership. I returned the car within a few days and complained about the same noise just as you described. They told me not to worry about it, that it is common in these vehicles. I was skeptical about there statement but I now have 54000 miles on it and have not had any trouble. I’m not suggesting that you should dismiss the symptoms, just saying what I experienced. I have not experienced the “humming accompanied by vibrations” you mentioned.