You should also check out my recent post on the same topic in which benz47 was very helpful. Search for "humming noise over 50 mph". Anyways I have had this problem and through peristance and two different "Five-star" service dealers as well as the information from this forum I am getting my differential replaced Monday! Thank the lord! Go to the dealer, know your stuff, be persistant, but most importantly get a test drive with a mechanic. Thats what convinced the dealer to fix it for me! Its a manufacturing defect and it does get worse! Mine already has since I bought it! The humming is noticeably worse. Dealers might try to tell you its normal, but its not! I only had my 7 year/70k mile warranty and replacing the differential is covered so all I will pay is 100 dollars. Best of luck to you. Don't be intimidated by the dealers it will be worth it in the end.