If your vehicle is showing high-pitched whining or buzzing noise coming from the alternator, here are the most likely causes and the parts you'll need to fix each one.
The internal bearings supporting the alternator's rotor wear out over time, producing a mechanical whining or grinding noise that gets louder as engine speed increases.
When the internal diodes fail, the alternator produces an electrical 'magnetic' whine or buzz. This is often accompanied by dimming lights, a dead battery, or a battery warning light on the dash.
A worn belt, failing tensioner, or idler pulley located right next to the alternator can produce a high-pitched whine that is easily mistaken for the alternator itself.
If the serpentine or alternator belt was recently replaced and tensioned too tightly (especially on manually adjusted systems), it puts excessive side load on the alternator bearings, causing them to whine prematurely.
A dying battery forces the alternator to work at maximum capacity constantly. This extreme electrical load can cause the alternator to emit a loud magnetic hum or buzz as it struggles to keep up.