The clutch release bearing is a thrust bearing that absorbs lateral loads, i.e., loads that are perpendicular to its plane of rotation in your car.
Put another way, this means that when the release bearing is brought into contact with the rotating pressure plate when the clutch pedal is depressed, the rotating part of the bearing absorbs the pressure of compressing the diaphragm to remove the diaphragm’s clamping force on the driven plate.
In most cases, the release bearing will last as long as the rest of the clutch assembly, but given the facts that release bearings operate under high-temperature conditions for extended periods and that it has a limited supply of lubricant, a release bearing can sometimes fail before the driven plate’s linings wear out. Here is how to recognise the symptoms of a failed or failing clutch release bearing.
Loss of lubricant or the ingress of dust and/or water into the bearing can damage the bearing to the point where small pits or irregularities are ground into the bearing’s internal surfaces, which is what causes the whining noise when pressure is placed on the bearing.
While a slightly damaged release bearing usually causes a whining noise, a severely damaged release bearing often produces loud, grinding noises when it is placed under pressure. In some cases though a severely damaged release bearing may seize or disintegrate, which usually results in fatal damage to the pressure plate when parts of the pressure plate that is contact with the seized/disintegrated bearing are sheared off.
As a practical matter, it does not matter what noise a clutch release bearing makes when it is under pressure. Since these bearings are supposed to operate silently, any kind of noise is abnormal and serious damage to the clutch and/or transmission could result when the bearing fails catastrophically. So if you are experiencing any of these signs, see a qualified mechanical workshop.