The front bearing is still a pressed-in design that has to fit tightly in the spindle. It's hard to believe the new one is loose because it would be wobbling around and spinning, and that would continue to eat the spindle away. You would hear knocking and rattling, and that wheel would wobble around when you grab it and tug back and forth sideways on it. That wear would continue to worsen over time, but that alone won't cause a hum. The hum comes from tiny dents in the bearing "races". Those are the two smooth hardened steel surfaces the steel marbles or rollers ride on. Those dents develop from an impact such as hitting a pot hole really hard, or when the bearing isn't held together tightly with the axle nut.
If the bearing assembly really was loose in the knuckle, both mating surfaces will be very shiny from the polishing action the parts created when they moved across each other, or they will be rough and chewed up if there was excessive movement taking place between them. Normally the bearing and the hole in the knuckle will be a dull gray or silver color and there will be very small scratches going straight in and out from the previous removal and installation of the bearing. Those are some clues to tell if the bearing assembly was loose in the knuckle. There is also going to be a plate that bolts on to hold the pressed-in bearing from working its way out. That plate will have a shiny ring where the bearing ground the surface rust off if it really was loose. That still would not cause the humming sound though. It's hard to say for sure, but for such an early repeat failure, I would be thinking the axle nut wasn't tightened to the proper value. A lot of do-it-yourselfers set the car down on the tire to hold the axle from spinning so they can tighten that nut with a torque wrench. At that point the damage has been done to the new bearing.
For the noise to not return for a year suggests the nut was tightened but not enough or the old axle nut was reused. Your bearing comes with a new nut. Some of them are a "torque-to-yield" design which means tightening it stresses it to a calculated, designed-in amount. That can only be done once. When that type of nut is reused, you MIGHT get lucky and not have problems, but it isn't worth taking the chance.
Monday, April 11th, 2011 AT 6:38 PM