Failure and noise of the alternator

Alternator failure and noise.
This is the sound that was in the drive recorder. During operation, the noise is just not heard only a little.
Alternator alternator noise frequency was higher Once the failure has been changed to sound like a low harmonica.
When the three-phase AC power generation part is only no longer power generation two-phase appears to be in this kind of noise.
Although we are somewhat of power generation, it was stalling and remove the negative terminal of the battery during the idling (when the normal did not stall).
Please help us to early detection of failure.
Alternator noise may be heard from the radio, but this video which was mixed from the power supply line to the drive recorder, do not most heard during operation. But when the alternator is broken we found that broke because has changed engine sound.
Car Model: NISSAN CIMA Y31（1991）