The most common cause of vehicle whine is a loose or worn fan belt. This could be the result of a loose tension wheel or simply the belt itself being old and losing its elasticity.
The same belt drives many separate components of a vehicle, including the alternator, the air conditioning compressor, the water pump and the power steering pump. Any of these components could also be the source of the whine if their bearings have worn down. Likewise, the transmission and drivetrain have bearings, and if the sound only occurs when the car is in motion or only when changing gears, one of these is the likely culprit. Finally, brake squeals are not uncommon, as brakes that have worn unevenly or that have developed burrs can cause the brake drum to squeal noisily when the driver attempts to stop the vehicle. In all but a few cases, these repairs are inexpensive and relatively quick to complete. Of these, the most potential danger comes from worn brake pads, which increase the distance required to safely stop the vehicle. This danger can be compounded by poor road conditions and can lead to a serious collision if it is not addressed promptly.