Well, I rotated the tires and the humming noise was gone. But a bigger problem showed up. My whole car was shaking at speed 35mph. Then, I took all 4 tires out to inspect them and found out that one of the tires was bad. Since I had over 40,000 miles on these tires, I am changing all 4 of em. I will post the result about humming noise after i change the tires today.

Thanks for your feedback.

-Bill