Chevrolet Repair: Humming noise coming from 2005 Trailblazer

Question
2005 Chevy Trailblazer LT
4 WD w/ 5.2 liter engine

I have a 2005 Trailblazer with 81K miles. It is the extended model with 4 WD. About 2 months ago I put two new tires on the rear and I started to notice a humming noise that started around 60 mph that I had never heard before.  It is most pronounced at 65 mph. When I decelerate below 60 mph it goes away. There are no other symptoms (ie. noise, vibration, or shaking). I had the tire folks drive it and they said it was not the tires and thought it was something mechanical. Any ideas?

Answer
HI DOUGLAS, I WOULD DRIVE THIS ON A SMOOTH ROAD GOING STRAIGHT AND PAY CLOSE ATTENTION TO THIS NOISE. NOW AT THE SAME SPEED YOU HEAR THIS NOISE TURN SLIGHTLY TO THE LEFT AND RIGHT. IF THE NOISE GETS LOUDER OR ALMOST GOES AWAY YOU HAVE A FAULTY WHEEL BEARING..USUALLY FROM THE FRONT...THIS NOISE COULD OF BEEN THERE FOR A WHILE BUT THE BAD TIRES WERE CHANGING THE SOUND ON YOU..NOW WITH GOOD TIRES YOU HEAR THIS NOISE...IF THIS IS A BEARING NOISE A GOOD REPAIR SHOP WILL BE ABLE TO TELL YOU WHICH BEARING IS BAD...IF THE ABOVE HAS NO CHANGE AS I SAID ABOVE TRY TO MOVE THE FRONT TIRES TO THE REAR AND SEE IF THIS CHANGES THINGS..LET ME KNOW AND GOOD LUCK TO YOU...TOM