Alternator failure is a common issue on these 99-04 Mustangs. Not sure why but alternator just starts going out, you’ll know when you start getting that flashing battery light on your dash when driving. Luckily, my alternator started making a high pitched sound (it never completely died though) so I ordered a new one (not remanufactured) right away and it fixed everything. Speakers and sound system started to sound like it was all blown due to low voltage from factory alt. Lasted 87K miles.
Nothing to add but out of curiosity where was the high pitched noise coming from? Both times the alternator failed on my Corolla a high pitched sound came from the radio.
Just curious, but I’m wondering how you determined that alternator failure is a common problem with those cars.
The high pitched sound is most likely due to excessive ripple voltage on the power bus due to bad diodes inside the alternator.
It can also come from worn out brushes making and losing contact with the armature if the noise is coming from the radio. If not from the radio it can be a failing bearing in the alternator.