There's a friend of mine who has a car (2007 Suzuki Swift petrol 1.3L DOHC 92hp) which has been making a whining noise for a while, the alternator belt broke once few months ago, and during it was broken, there was no noise anymore.
When I checked, I found that the loud noise came from the alternator with a smell of burnt rubber, it really sounded like slipping belt, and it started 10 seconds after turning on the engine on the cold, or sometimes when turning on some electrical features like headlamps or when maneuvering at low speed. Thus, there was a load of rubber particles all around the alternator pulley, when it started to whine, the temperature of the pulley rised very quickly, and tension felt from 14.3V to 12.8V when it whined.
I tried to replace the alternator and belt but it didn't improve. I also tried to unplug the alternator, both the 12V and the control cable, but not at the same time, and in both cases, it stopped whining (and charging).
With all I observed, I assumed that I faced an electrical or electronical issue. Some component may need much energy, and to fulfill the needs, the ECU might increase the coils' impedance in order to produce more power (and by too much increasing it, it makes it hard to drive), but I don't know which component is the cause of that. The battery seemed okay though it was old, so I tested with my battery which is 3 months old, full and more powerful anyway, and it still whined.
Which component could induce such a problem? Could it be a bad ground? If you have any suggestion, I'll appreciate it.