Ok... stopped by my dealer on the way home. Brought up the rear humming sound to my service rep.

I couldn't believe his answer.... he asked me if I noticed the humming only when I had a passenger... and suggested that I was "overweight."

Apparently the max weight rating of the passenger compartment is 415lbs. Which is something I didn't know... but STILL!!!!

It was kinda balsey of him to imply I was over the max with a passenger in the car.

Now that I'm done ranting.... and I think back on it... I usually notice it when there is someone else in the car to complain to...