Clutch release bearings are designed to release the clutch by applying pressure to the pressure plate fingers. To help them do this, they are lubricated before being sealed within their container. Over time, this lubrication can leak out. Without that lubricant, the bearings can’t turn smoothly and begin to wear out and become noisy. When the release bearing is worn you'll often hear a rumble or squeal when you step on your clutch pedal. Usually, the clutch release bearings and the clutch disk are replaced at the same time, purely because it's such a big job seperate the engine and gearbox.