2006 Volvo XC90 wobbling noise

Hello, I have XC90 2006 2.5; Since January 2010 I hear wobbling noise coming from rear of the car. The sounds comes out when the car is in motion and it is well heard on low speed, when accelerating or in higher speed wobbling is that fast that it appears to blend with background noise and cannot be heard that well. The car was diagnosed in CarMax but nothing had been found, I was told that it is probably wear on the tires. Tires are in good state though, the sound comes out alse whe tires are rotated. I have also change out wheels for testingand still the woobling sound comes out. Wobbling comes out definetively in syncronization of rotating wheel, and can be heard also when vehicle is stopping - when applying breaks. The wheels were balanced and technicians check the bearings and nothing could be found. What could be a problem?