It's not the Rear Side Belt (going up to the Compressor), since that was replaced.
After replacing the belt, there is still a sound, albiet slightly different. Now there is a definite "wooshing" when turbo is spooled.... it's a leak. I think, at least.
The dealer ordered new gaskets for the TMIC, which they will install when they come in next week. Hoping that solves it. Another tech remembered a similar problem on a newer car and that did the trick. We'll see.
Got a free belt and install out of it