Had another chance to revisit this today, the rumbling is almost certainly coming from the alternator. I was able to very this by using a long screwdriver as a "stethoscope" and the noise is far and away loudest when placed on the alternator compared to the waterpump or AC compressor areas.
What is my path of least resistance on fixing this? New alternator? New to me alternator? Is it possible to just replace the pulley? I'll be doing this myself so any advice is much appreciated.
Here's what I have:
