Spent some time on this issue with a friend this afternoon.
We used the dowel as a stethoscope trick on the alternator and heard rumbling.
But then we tried it on the AC compressor while disengaged, and heard rumblings there, too.
So we're not a lot wiser.
We tried to remove the drive belt to spin shafts individually, but the pretty sketch in the shop manual is wildly optimistic.
We see
https://photos.app.goo.gl/cQmMSYUs9FB7BacHA.
The nut-shaped boss on the tensioner is recessed under the end of the head,
https://photos.app.goo.gl/S9fs2FeLNFbw3jEE6
We got a 19mm box wrench on but it needs far more force than we were able to apply.
We could not get a ½ inch breaker and socket in there. A friend has some flat tension tool he's offered.
Plus it looks like putting it up on the lift may well make it easier to get a new belt on there.
(Maybe you can see more from underneath but it looks like anything besides the alternator requires pulling the engine.)