Stop that whining

I BOUGHT a new Mazda6 wagon in 2002. A whining or slight knocking noise comes from the rear and usually occurs about 60-70km/h. It has been there since I bought it. It sounds like the bearings. I have asked my dealer to check this each service, but I was first told they could not hear it, then that it was tyre noise from slight wear in the inner of the rear tyres. I have owned about eight cars and never heard a noise like it, especially from a new car. The warranty runs out in September and, if the problem persists, it may be expensive to correct.