Wheel bearings are designed to keep a vehicle's wheels spinning smoothly. When they fail a growling noise, and a loose hub assembly will occur. A sealed wheel bearing has its seal incorporated into the wheel bearing assembly itself. They fail and become loose and noisy due to contamination and a lack of lubrication. The first symptom of bearing failure is a growling noise coming from the wheel associated with the bearing. If a sealed wheel bearing is noisy or defective, grasp the tire at the top and bottom and rock it back and forth. Looseness indicates a bad bearing.
They are not serviceable, most of the time they're pressed out of and into the hub. Others are easier to replace because the hub and bearing are removed and replaced as a unit. If there's end play, inspect the wheel for looseness or a worn wheel bearing. End play is the amount of horizontal movement in the wheel.
A tapered wheel bearing is different than a sealed bearing. It may or may not require a bearing preload. These bearings are serviceable and should be cleaned, inspected, and packed with bearing grease. Always check wheel bearings for excessive wear such as galling and abrasions.
They will appear dark colored or turn dark blue when overheated. This overheating can happen if the bearings where over-torqued or lost vital lubrication. All wheel bearings are especially susceptible to water and dirt contamination. To prevent grease leakage, inspect the wheel seal, and make sure it's in good condition and has a tight fit around the wheel spindle.