The clutch release or throwout bearing depresses the pressure plate fingers, releasing the clutch. It contains a bearing that spins along with the spinning pressure plate as the clutch pedal is depressed. If this bearing is worn, a squealing or growling noise will be heard when applying the clutch pedal. A noisy release bearing will be heard through the transmission’s clutch housing.
Most of today's manual transmissions have hydraulic clutch circuits. Still many have mechanical linkage; they use rods and cables to control the clutch. They have constant running bearings that spin whenever the engine is running, and must be adjusted to maintain a slight preload after installation. The engine and transmission must be separated to replace the bearing. Typically whenever the clutch disk is replaced, the clutch release bearing is replaced as well.
Bearings begin to growl as they wear or loose lubrication. A throw out bearing is no different; its lubrication is sealed inside. It's typically necessary to lubricate the shaft the bearing rides on during assembly. Check in the manufacturer’s manual for assembly specifications.
Listen for a chirping sound as the clutch pedal is first depressed. This is the worn (un-lubricated) release bearing contacting the fingers of the pressure plate’s diaphragm. These fingers can also wear causing similar noise related issues.
A standard release bearing is separated from the pressure plate. It's adjustable and should not contact the spinning pressure plate when it’s released. A constant running release bearing snaps into the pressure plate fingers and spins whenever the pressure plate is spinning. Because it’s attached to the pressure plate, if this type of bearing is faulty, it will make a growling noise whenever the pressure plate is spinning.