We have a 1990 GMC Suburban with 4 wheel dive. Right now, we have our road tires on it. Lately i have noticed a noise when the engine is lugged a little bit in over drive. the noise sounds kind of like a train, while standing by the tracks. (sory, but thats the best way i can describe it) The tires were balanced when they were put on the wheels, which was just last year. I believe that the noise is coming from the bearings at the front wheels. Anybody got an idea of what it could be?