I think that it might be in partial lock up as a result of faulty diodes. The same weird failure and skreeeeech sound happened to me as the alternator failed. It was the belt squeeling as a result of the alternator drag. I pulled the alt had it checked and sure enough the diodes were faulty. Pull yours and have it tested on the bench. OEM has subs in foreign places building there alternators. These days it's hard to get quality.
To solve my alt issue after trying 3 from local suppliers I got one from a local rebuilder that uses better quality parts that solved it.