Turns out it was the alternator. Guess how I found out?
Lesson 1, don't wait too long once you hear the rumbling.
Lesson 2, don't skimp on an eBay alternator as a replacement. I should probably be glad that it broke before I got it installed in the car.
Ended up buying a new Bosch and everything is working amazingly. Thanks so much to everyone here for helping me navigate this problem from start to finish!