My wifes 03 Explorer had that same problem. We replaced all 4 of her tires within a week I started noticing a noise coming from the front of her car until you reaching 45 mph; it was the humming noise like the mud tires sound. I thought it might just be the new tires and we needed to get adjusted to it.

A few weeks later she was run off the road and had a blowout on one back tire and when we took it to get replaced we found out the front two tires were cupped and had to replace those as well (under warranty). I mentioned the noise becasue I thought it was the tires making the noise but the mechanic said it sounded like the bearings were bad. We took the car to the dealership and found out that not only did the bearings have to be replaced but the problem is what caused the front tires to become cupped!